Servers crash at start of boss wave

This just occurred on Otaku's Corner. It seems I run into this every few weeks. As soon as the trader time before boss hits 0 the server seems to crash and everything just lags (people running into walls). Everyone gets booted from the server. You can reconnect and the server will be online (albeit on a new map with no players).

Is there a known cause or trigger for this to occur? It's always a bummer especially when you have a good group going.