I think people (myself included) get on Marshall for a variety of reasons.
1. He's an easy and obvious target. He came on the board loud and attacking, and people don't like that approach.
2. His choice of people to attack seems odd. He goes after Barry, Joann, Joshua, and Al. These are people generally considered to be even tempered and likable. If he was just rude to Bruce, or a couple of others, people would view him differently.
3. He's a square peg trying hard to fit in a round hole, and I don't just mean the board, but life in general. Guys like him were funny back in college, but most grew out of it.
I could probably go on for a while, but anyone asking why we're "quick to jump on a guy like Marshall", is just spoiling for a fight himself.