We do not know the exact mechanism that causes it but it happens all the time. Something in the parts file can corrupt the entire parts database and the only way to fix it is to delete your entire parts library and reinstall Fritizing. When this happens it will often stop Fritzing from loading or causes the parts update to fail.
As for parts submitted on the forum they are often broken and as you say unverified. If a part is submitted to the github parts repo and gets excepted that means it has passed testing to be compliant with Fritzings internal standards but it does not get verified as being an accurate footprint or correctly labled etc. It just get verified as being compatible with Fritzing.
Fritzing can be very slow at doing certain things and will become unresponsive for very long periods of time. This is not a sign it has crashed or that something is wrong. You often have to walk away for long periods of time (30minutes+) to let it do its thing and finish. If it is closing on its own then that is a problem but you have not explained how it fails for you.