I don't think sar provides that info.
Its a little difficult to post-analyse stuff like that if you weren't already setup to record the relevant info.
However, if you search through the various logfiles (/var/log/...), you should be able to see what was running in that hour.
That will give you a start.
There are various monitoring tools available eg http://collectl.sourceforge.net/
A quick way of finding out next time, is to setup top to run (in advance) in batch mode http://linux.die.net/man/1/top