Well, if a program terminates unexpectedly (i.e. it crashes or is killed), the icons that were put to the system tray by this program stay there - because the program didn’t instruct Windows to remove them. When you move your mouse over the icon, Windows try to send the corresponding messages to the owner program, finds out that the program doesn’t exist anymore and removes the icons.

So, the thing means that the avast! process responsible for handling the icons (was) terminated for some reason… it would be nice to find out why, of course ???