Actually I have an idea about the intra-colony attacks, when research was first done on the Argentine ant (Linepithema humile) invasion in the USA it was thought that they all belonged to one giant supercolony, however we now know that there are a few super colonies in the United States and that in their native range they form normal colonies with normal colony aggression responses.
Interestingly there was some work done on some lab colonies of Linepithema humile and in the same lab some roaches were being bred/studied, so to save money they of course fed the roaches to the ants! Makes sense right?
Well after feeding the ants on the roaches the ants that had been feeding on the roach were attacked by the other ants in the colony! As if they were from a separate colony, very interesting indeed as these ants were from a USA supercolony and should have shown no aggression even to ants separated by miles....
As it turn out the roaches happen to have on the exoskeleton some complex hydrocarbons (I think that's what they are) the same kind that ants coat themselves in to make a specific colony odour, when the ants fed on the roach these hydrocarbons rubbed off and made them smell like ants from a different colony thus they were attacked, though the ants being attacked did not retaliate as they still recognised their nestmates colony odour.
Perhaps a similar thing is happening to your colony? Though it does not sound like there is any danger at all to your ants so I wouldn't worry about it!
Lol, sorry for the long explanation but it was all relevant!
Oh darn, Adam beat me to it
Remember, karma. So be nice! =P