Thanks for the update.
In our sitation, I believe it was related to some STP issue somwhere else in the network.
Just before the mac-addresses were dropped, we could see that "show spanning-tree bridge detail" updated the "time since last topology change" to 0 and increased "number of topology changes" by 1.
I guess an STP-change could lead the the switch wanting to re learn the mac-addresses...
But if the problem turns up again we will remember to look at this setting as well.