It was a mission reload, for whatever reason, by a player named "zet" from what I've seen so far. Here's the lines that lead up to the massive drop-off of everybody.
15:59:02 ca\missions\MPScenarios\MP_Deathmatch.chernarus: string @str_mpdeathmatch_subname cannot be localized client-side - move to global stringtable
15:59:03 "DISCONNECT START (i): __SERVER__ ("") Object: <NULL-object>"
15:59:03 "UPDATE: [<NULL-object>,[],true]"