This article was originally posted in June 2011, but contains seasonally relevant information.
Whenever there isn’t a good flow on (like now and throughout the rest of the summer) strong honey bee hives will often rob weak hives – if it gets bad enough they will completely decimate the hive that is being robbed.
This is the best video that I could find that actually showed robbing going on. Notice 2 things 1) The robber bees are climbing up the hive to get some extra elevation before they take off – this is typical in a robbing frenzy. 2) Groups of bees wrestling on the landing board, and falling off the front in clumps – those distinguish a robbing frenzy from orientation or swarming.