Why would the electrician use 8 gauge wire on a 20A circuit?
There can be many reasons for using a larger than normal wire, including but not limited to.


The person desinging the installation thinks that the breaker rating may need to be increased in the future. 
The circuit is unusually long and the larger wire is needed for volt drop reasons.
Thermal insualtion or grouping concerns require the cable to be de-rated.
The electrican was in a hurry and simply didn't have any smaller cable of the appropriate type on-hand.
Someone simply made a mistake.