He probably meant to take on c3 and then play Bd2, but played the second move of his analysis first. I've done this as well:
In my first 3 rounds I had draws with GMs van der Sterren and Nijboer, and IM Remlinger. Now I'm playing someone 400 points lower than me.
I wanted to play ...Qb2, and started looking at some variations to make sure my queen wasn't going to get trapped in there somehow. One of the lines had me following up with ...d5. Satisfied with this, I played ...d5 right away, and resigned after Nxc3.