Probably simply as they were in the area and figured they'd never get the opportunity again, I'd guess =P
I felt it was fairly funny, though they needed to make the sound effects sync up better. Less random noises and more carefully trying to mimic whatever causes each effect. Also for best effect they needed to have Mario fight Sonic or something ridiculous before they began using his Brawl taunts.
I'm wondering, did they have to procure the Sonic costume at the time, or did they happen to have one on hand? I would not be surprised to find that they keep a number of costumes on hand wherever they go, just in case the situation was ripe for parody. The entire video was spot on. One of their finest.
Procure most likely. remember they don't like sonic soo much and they don't do any skits of them unless its to make fun of sonic so I doubt they would keep a costume on hand like the others.
I'm pretty sure they said on the podcast that they bought it (or most of it) somewhere. Wouldn't be surprised if Garrett made the whole costume himself out of paper clips and an old tuxedo or something, he's literally the IRL MacGyver.