First, you should read
this.
This type of donk bet is typically, "I've got TP, but I don't want to give the FD a free card." Donks always assume that someone has the nut flush draw, even though the chances are pretty low on a random basis. If you call and the FD completes on the turn, he'll c/f. If you raise to say 80, there's $160 in the pot and he's only got to call $46. Nobody is folding TP in this situation getting nearly 4:1. Since he doesn't have any more anyway, he knows he doesn't have any FE.
The other issue is the SB. While he probably has nothing, you don't want to give him direct odds to call with any of his possible draws.