CCs can close for many reasons. You got the primary one right - lack of attention. Use 1/2 of your cards every month, keep your balance under 10%, and rotate them. That's the "FICO Rising" game.
Second - being over limit (usually more than once)
Third - late payments. They don't like chasing down their money.
Fourth - hopefully soon to be extinct - UD Universal Default. If they see you've maxed out other cards or even hit over 85% utility, they can consider you a high risk and close you.
They should have sent you a statement giving you 15 or more days notice as they were the ones to close the account. (sometimes they forget to).
On the bright side - if they don't want you - you don't want them.
11/14/07 TU 567 - EQ 561 - EX 577
11/29/07 TU ??? - EQ 573 - EX ???
12/10/07 TU ??? - EQ 579 - EX ??? - 60% util - OOPS!