I have had this happen several times.
Usually right at the optional when everyone is need/ greeding. Then you get the message left team channel, then everyone passed on x.
Cannot reloot it as it is still greyed out. Cannot rejoin team.
Again, I am playing KDF and this is happening to me. I am not leaving the group, I am being removed somehow. When I ask what is going on, no one says anything. I have started writing down the names of the team members to submit to the Devs. It keeps happening when rolling for optional loot in ground missions. Auto passes on rolls even when picking need/greed.
This is some kind of bug. It doesn't so much kick players from a group, but more disbands a group, or at least I think it does. It happened to our group several times yesterday, at various points during a number of STFs. Sometimes one of our group reinvited everyone, and sometimes we continued just using local or zone chat. Regardless, every time we did not have a problem with loot.
Oh, and this group I was with was usually all Fed, although it did happen a couple of times with KDF players in our team too, so it's not just KDF players trying to ninja themselves a bit of extra (pointless) loot. Have you seen the loot in the STFs? Why would anyone want to ninja it? Having said that, it doesn't stop people rolling Need on junk they are just going to sell.
Yeah, I think the group disbands because of a bug. Happened to me a couple of times, and with people who I know aren't trying any shenanigans to steal the loot. Plus both times it was on the boss, so the optional had already dropped, so it wouldn't have worked to steal loot anyway.
AFAIK, the teams for anything you do through the PvE queue are automatically formed when the STF/Fleet Action starts anyway (I think the same applies for private matches, but I'm not 100% sure), and you can't be kicked by the team leader or leave them unless you quit the STF, so it's a bug.
If it happens to you in a PuG, just invite everyone else in the STF to team.