Problems starting apps on the iPhone 3GS

I had suddenly a very strange problem. While being in the video app, the phone suddenly hung up, the boot (?) apple logo appeared, nothing happened, a few seconds later i was in the unlock screen. From that point on I could not start any downloaded apps anymore (evernote, tap tap revenge, twitterific and so forth). I've resetted the settings to default, this haddened helped. Then I uninstalled one of those apps, downloaded it again from the app store and now everything runs smooth again.