Cracked Exhaust Manifold

This can happen from fatigue as they shouldn't have had to touch your exhuast manifold, though it is possible they did. Some shops perfer to remove the exhuast to do a catalytic converter, others will just lift the car and brerak loose with a cutting torch. It really all depends on the shop's policies and procedures. Have it check by another mechanic for a second opinion because really, both could happen.