Thankfully, Twitter underwent a massive cleanup the other day to purge the system of spammers and suspicious accounts (story here). This is something I wish they'd do much more often, but out of nearly 850 followers, I only lost 4.
It's easy to find quick references of Twitter users wondering why they lost so many followers over night. "Was it something I said," is posted quite frequently.
Those that lost enough followers to wonder what happened are missing the point of Twitter. They didn't even know who the people were that were following them. If you aren't building relationships with followers, you aren't networking – you're talking about yourself.
If you are mildly interested when you see someone promoting how to get 400 new Twitter followers, ask yourself what you intend to do with 400 additional followers. Likely, if that's important to you, without knowing who they are or what sort of background they have, you're simply selling something less useful than 400 additional followers.