An account subscription expiring, as far as I know. So when your subscription ran out before the pact was broken.
Will that happen even if you don't log in the characters that are pacted? So 1 & 2 are pacted together. 1's account goes out of credit but I don't log in 2 until 1 is subbed again.
Yup, from what I understand, as soon as the account expires it's bye bye levelling pact
Damn 
Well that will really encourage me to re-sub my accounts then.
Back before last I quit the game I had several duos in levelling pacts. Since I returned I have played all of them. But when I logged back in on them they all had the message in the chat box 'pact cancelled' (or similar). At the time I only had one of the two accounts paid for.
Question: What acts other than user intervention cancels Levelling Pacts?