There's absolutely no way to tell without more details on what you're doing when they happen. That said, an allocation problem means you don't have as much free memory available as TPT is trying to use. Now, this doesn't mean that you're actually low on memory, it could very well be a bug that makes TPT want to use insane amounts. If you've customised your undo history limit, what value is it?

There you go. It defaults to 5 for a reason :P