For me, it happens when I switch from another logged-in Ubuntu user to myself. Login goes OK, but then, when building the screen, I already see in the system monitor I have running in the top panel the amount of memory going up due to Skype trying to start. Memory goes to 100% for some 20..30 seconds, and then Skype dies, making the system responsive again.

It does not seem to happen when I start Skype manually (and also does not happen always). Don't see anything special in logs, but also don't really know what to look for.

For the moment, no auto-start for Skype...