This isn't a very common thing, but there are documented incidents where guardians are kicked from fireteams just before a boss dies. This is usually done so that the fireteam leader can (a) bring in someone they know to share the rewards, or (b) enjoy knowing that they made someone work and denied their rewards. A solution might be to prevent kicking when an encounter is in progress, or when a boss starts taking damage (or a combination). Thank you 😁