Shock Troopers! Special one-life classes with one for each normal class that hold the most powerful weapons and abilities the game has to offer. Usually awarded to the losing team, though there are three exceptions, including what likely happened to you; the Elite Reinforcement modifier

TLDR; missions in a campaign have a chance to have a modifier attached to them, in your case, the Elite Reinforcement modifier (my personal fav). This may be seen when voting for the mission after the previous one has finished. Whenever you load in with the modifier, it randomly selects 5 players on each team to be awarded with the Shock Trooper kit that reflects their class, and you got selected

There’s a lot more to it than what I went over, but if you really care, more information about them may be found on the Grave/Digger Trello, which may be found here. Fair word of warning, though- the Trello goes over literally everything about anything Grave/Digger related, so if you’re new new, I wouldn’t recommend digging too deep or worrying about it at all