I believe it is the Bad Juju bounty causing most of this. It's a common bounty and requires you to do 25 Strikes as part of the objective. Unfortunately you can't solo Strikes, as you're automatically thrown in with other people. Most people who do this bounty, myself included, just want to get the Strikes out of the way. If you want to kill enemies for Legendary items, there are more efficient ways of doing that than fighting enemies and bosses your level.