Just inside the gate is a trip wire linked to a bunch of grenades, you failed to notice this when you opened the door and walked right in setting it off. You fail to disarm a trap you only have yourself to blame.
....ITS A TRAP!!!!!!!!!! :nuke:
Just got to be aware of your suroundings and disarm the traps. Either that or run into the tripwire/landmine/grenade cluster/rigged shotgun/weighted deadfall before your companions do.
The confined space of the room and the tunnel make it a very deadly trap the grenades have no room to spread out. Rushing in ahead won't save the companions if their near by. Only disarming or leaving them further back in wait mode will do it.