..?! That shouldn't be possible, unless the admins have something enabled that makes you lose all upgrades after dieing and get them returned in points, so you can make a new setup everytime you respawn.
Or they left the server, joined again and chose new equipment. I did this once, when I realized that a grenade launcher was completely useless on one map.
Comments
Or you can use an extra level to change gun. (Ie, go HMG, rack up a few kills, select Nade. [It does waste a level.])