That would be from a slap on the behind by a "Charlie's Angels" crew member, TMZ.com reports. Kelly apparently was prepared for the situation:

TMZ also said that Kelly didn't want the man to get fired, but that ABC has a zero-tolerance policy. Which might be quite the contrast from the tales from the "Angels" set in the late 1970s, considering a whole TV movie got made about the experience.

Also, the TMZ report says the crew member has had previous run-ins. Which probably means it was clear that he was not trying to celebrate with Kelly like her ex Derek Jeter does with his teammates: