AT&T suspends one of our 3 lines without us asking...WHY?

Let me guess, you used the SIM in the repaired, blacklisted iPhone you found? 

Yeah. 

That happens.

Dont use that phone with a Sim.  You have to call AT&T with a valid IMEI to reactivate the SIM card.

Only AT&T knows. No one here has access to your account. You will need to call AT&T for reason.

A bit more information might help us guess.  Likely it's phone related.  Like the SIM was used in an unauthorized device.