I-Pod Touch 8GB Will not Run User-Installed Applications

Now, this isn't really a question as I have solved it myself but I am simply curious as to the cause (and probably to help others who may have had the same issue as I didn't see this type of question floating around already)

I got an I-Pod Touch Brand-new from the box two days ago, was running perfectly fine with the few applications I had installed on it. Just today I ran "PDF Viewer Lite" from the App Store, did as it should but however a couple of minutes later any other applications (including PDF Viewer Lite) would not run. I confirmed they were running in the background -- applications like ICQ or AIM were still displaying notifications to messages but would not display themselves when clicked as they would simply show their main screen for less than a second and boot me back to the main menu. Hard resets did not work (HOME button at bottom + power button on top being held), and even a restoration from backup had failed. Before I restored the I-Pod to its factory settings, I reinstalled the offending program, PDF Viewer Lite and automagically, it, along with everything else, decided to work as they should.

So my real question is, what could have caused it? I've used PDF Viewer Lite just yesterday and it didn't lock up all of my applications on my I-Pod. I'd like to know what caused it so I can avoid it in the future -- as I'm not always connected to a WiFi network to reinstall that application.