Wiseau's money
One source of mystery surrounding Tommy Wiseau is how he came to be so wealthy. In The Disaster Artist, Greg Sestero stated that at some point Tommy was in a near fatal car crash. I'm thinking that accident may be responsible for both his unique demeanor and his vast fortune.
It's possible the accident caused severe head trauma that altered his personality and/or made it very difficult for him to remember things (such as lines or important plot points). It's also very possible that the accident resulted in him receiving a large personal injury award. I find it hard to believe he made that much money selling imported jackets or bird toys and think his accident may actually explain a lot.