It's definitely a whole combination of factors. How you que, what time of day, which class are these players, BGs is an endeavor, and MMR issues.
I play BGs often and when queuing solo during the low traffic parts of the day I can hit kill numbers like that pretty frequently. This is usually due to being teamed up against less experienced/more casual players. For me this changes during peak activity hours when there are more PvP guilds and experienced players on. Don't get me started on the group que and playing against premades.

As other have noted it is easier for some classes to get the final hit in and it counts as their kill. I run a more melee brawler style build and there are many times when I can get a player down to low health and then a sorc or a nightblade can quickly fire off their execute and they get the kill. So if the other players on the team have a lot of assists but less kills this could be why.

If BGs is an endeavor, then it can be chaos as many inexperienced players join and are farmed by the more experienced.

A lot of this does lead to questions about the PvP population and the game's MMR.