No it's not. Illegally holding on to property that's not yours isn't leverage - its theft and leaves you open to prosecution. If you have a contract with someone and they're in breach then the courts provide you an avenue for relief - not blackmail.

If this was his intention this is a really dumb strategy. He went from having a valid lawsuit for his bonus to loosing his company and being at the loosing end of a lawsuit. I don't buy him being that dumb. He just didn't think he was going to get caught.

i have never seen this behavior, so it isn't that common, and even if it were it would still be illegal unless it is something that has been agreed to contractually.