What causes a pokemon game to freeze up?

[Snowy]

I have a couple of legendaries i can't find so i thought they weren't transfered from my older games and went searching for them... When i tried to access the PC box in Pearl, the entire game froze... I couldn't even use the home button to get back to the 3DS' Operating system.

 

[Treacle]

Are all the pokémon on the game legit?

 

[Snowy]

I don't know. I received many of them in trade, but the game has never frozen like this on me before.

 

[skyradiant]

Your cartridge might just be really old, same thing happened once with my White cartridge (although in my case, it was because I kept resetting to get a Yanma swarm...).