Without a specific announcement, we really can't tell why.

I was on a flight where the pilot announced that she had seen minor damage on one of the jet engine turbine blades, but the damage could be fixed (that will buff out!).
After the fix, that engine was not only started at the gate, but run up to what seemed like pretty high revs. I was sitting more or less inline with the front of the engine so I wasn't too happy about what might have happened if the fix hadn't worked. But apparently the fix worked so we took off and completely the flight without incident.