Because no fraudster can prop it up indefinitely. Gotta lock in your profits before the crash.

like all ponzi schemes, it relied on new users getting sucked in. As other exchanges grew and Mt Gox doubts spread, I imagine their growth rate dropped.

Bitcoin has been pumped up and crashed 3 times. The fraudulent trading of Mt. Gox was only the most recent crash.

According to the actual post, it was at least the last 2 times.

Depends on what you consider a crash. IMO it crashed in November and hasn't had it's next climb yet.