Gossips and triangulates
Instead of telling a colleague they're annoyed, they tell you, then tell a third person what you said.
Rule out first
Gossip is ordinary social glue in every group. There may be no safe direct channel. A real problem may be getting reported the only way that feels possible.
What could explain it
Need to belong: The talk is mild, bonds the group, and they're friendly to the people discussed; nobody gets hurt.
Psychological safety: Most of the team does it, and the last person who raised something directly paid for it.
Triangulation: A two-person conflict always gets a third party: they recruit allies or messengers rather than speak to the person.
Machiavellianism: Information is traded selectively and timed for advantage; different people hear different versions.
Passive aggression: The gossip is the only outlet for anger they deny having; asked directly, 'there's no problem'.
A next move
Suggest they raise it with the person directly and offer to sit in. Willingness means it was a safety problem; relief and follow-through means habit; refusal followed by more side-channel talk means triangulation or strategy.
If it’s you
When I talk about someone, what am I avoiding saying to them?