What could be the cause of pain after having a crown recemented?

Three main reasons - two that have been mentioned below: high crowns and trauma from having the work done. 
The third possible reason is that your crowns came off because the root treatments themselves were flaring up, allowing bacteria to build up pressure under the crowns. Before recementation this would not cause pain because the gap between the teeth and the crowns allowed the gasses to escape. 
After recementation the teeth are again enclosed. 
An x-ray and gentle tapping on the teeth will identify if this is the case - especially on root treated teeth that should have no sensation at all. 
Good luck on getting this sorted -
Kind regards
Thomas Dey, BDS, MSc

Usually when a crown is recemented, it tends to be a little be "high," meaning you are hitting on it quite hard when you bite down.  This could cause extreme pain/sensitivty on your tooth.  It could also be sensitive due to trauma from having work completed.  My recommendation is to go see your dentist asap and let him/her know you are experiencing pain.

Hi:The primary reason could be bite needs to be adjusted and the secondary reason could be temporary trauma due to the dental work.  Best,Dr. Maddahi

There are several things that can cause pain after recementing a crown. The best way to answer your question is to return to your dentist for an evaluation.