This hand happened during our league tournament, how would you rule it?

Not sure of preflop action, (I was busy taking care of a rebuy at another table).
but blinds were at 50/100

3 players see a flop MP $2500, CO $1500 & BTN $300.

Flop comes 926 Rainbow

MP shoves $2500, CO tank folds, BTN throws his cards face up into the middle of the table, didn't say call & didn't but his chips into the pot. MP mucks his cards starts to rake in the pot, BTN says what are you doing I called. BTN had pocket 99 for top set so he was clearly calling. ( I believe BTN didn't realize he still had chips in front of him) MP had thought button had folded since he had thrown his cards into the middle.

Is BTN cards dead since he didn't say call or throw his chips in?

Throwing your cards away (face up or down) without saying anything and without putting chips in (not already all-in obv.) is usually a fold.

That said, it's a home game and he has the nuts, so for the good of the game, I'd just warn the guy but let his hand play.

Typically, by tournament rules in a public cardroom, if you throw your cards in the middle faceup, it's a fold. You didn't announce an action, and you're not allowed to expose your cards for a read or anything, so hand's dead, sorry.

In this spot in a home game, dude obviously just had a brain fart. Let him make the call for his last 300 and run it out.

That's was my first thought, but the MP was quick to muck his cards (he is a dealer at our local casino if that makes a difference, probably knows the rules). Would it be fair to him since the turn & the river hadn't been dealt yet. He never said what he mucked.

So let me just be clear about this: MP saw BTN turn up 99 and then immediately mucked his cards and started scooping the pot?

Honestly, sounds like maybe MP wanted to rush into the muck so that it would be a mess to play out the hand, leaving BTN without any real chance at recourse. And if so, then he kinda knew that the host might rule against him, which kinda makes this angle-shooting. Kinda.

It's a tough call. The fact that he's a dealer could go either way; he knows that mucking cards makes it tougher to deal with the situation, but he also might have acted quickly just because he's used to keeping the game moving. The fact that the hand is specifically a hand BTN would never fold makes me feel like it's the sketchier of the two options, though.

Anyway, since MP mucked his hand, if you want to play it out with BTN's hand live, tell MP to announce what his cards were, fetch them out of the muck, and deal it out.

If you don't want to do that (or can't), the hand can't proceed, so someone's hand has to be dead to decide where the pot goes. Here's the deciding factor, IMO: do you have a house rule about face-up cards being a fold / dead hand? If so, BTN's hand is dead, and MP takes the pot. If not, BTN's hand is live, and MP's hand is dead (because what he did is totally a hand-killer).

Moreover, if what I'm saying about MP's motivation seems to ring true to you, then you might want to have a word with him to make it clear that this is not acceptable behavior.

Home games are supposed to have friendlier, slightly more lenient rules. That's why they're home games. And in this case, btn had the nuts, and some other schmuck wants to treat that as some kind of fold? GTFO of my living room would be my response.

There are times when as a host we have to make a ruling that is in the strict sense of the law, even if it's unfortunate. This doesn't seem to me to be even close to that. GUY HAD THE NUTS. MP (and I agree with jim, this seems like as close to an angleshot as you can do without me being able to 100% say that's what it is) can tell us what 2 cards he had and I'll dig em out of the pile. (I would desperately want him to look at the board, realize 87 is his best chance and say that's what he had, only to find that there's no 7 in the muck.)

FWIW this kind of brain fart seems common. In one of my local rooms I have seen a couple of times where a guy turn his cards over when facing a bet otr, not announcing anything but he has a hand you'd think is a call (i.e. better than TP), and the dealer quickly stops the action to forcefully asks, "What's your action?" Doesn't always happen, really takes an on the ball dealer, but the spirit of the game is upheld, and afaik floor always has the option to rule that way.

Back to the OP, in case we forgot, GUY HAD THE NUTS.

In my games, the most important thing is to have the best hand win the chips when possible. Top set isn't a) a hand someone is going to ponder about or b) a hand where the player needs to "see a reaction" before deciding. Top set is a table pounding snap call of the all-in. NEVER ever is top set a folding hand. I think a fold of top set here could be an actionable offense in a tournament setting.

Top set hand is live. If I am feeling charitable, I'll let MP fish his hand from the muck. There are times I just give the pot to top set and call the hand over.

MP gets a "black mark" in my little book of notes. He gets a in private warning / discussion. MP likely knows better and is shooting an angle. I could entertain a defense that he is trained to handle this type of situation as played, but MP best hope I don't come to find out he lied to me about his casino's rules. If MP has a history of misdeeds, he either gets a "last chance" warning or the boot. If this is the case, MP is likely bad for the game and host shouldn't feel sorry about the situation.

Pffff -=- DrStrange