I have no idea why that's happening, but I don't think it could be caused by the theme... The Okuda theme doesn't do anything differently from the other themes (aside from covering a larger number of icons than most). No custom transitions, and nothing else out of the ordinary... Since no one else has reported any problems like this with the theme, I think there must be something else wrong on your phone. Did you install or change anything else around the time the crashes started?

If all else fails, I'd suggest backing up and flashing your N900. Sorry I don't have a better answer for you. :/

Do you currently have the Okuda theme selected, or one of the default themes?

Maemo automatically caches screenshots of certain app UIs, which it momentarily displays when the apps are first launched to provide the illusion of a faster loading time. Sometimes the screenshot it displays is from a theme which is no longer selected, which is what I think is happening in your case. It should work itself out on its own, but you're likely to see this occasionally if you switch themes regularly. Someone should probably file a bug on that (if it hasn't already been done), but it's kind of a tough one because the problem occurs intermittently and I don't know of any reliable way to reproduce it.