I often run into this issue, while travelling, or getting ready to attack a difficult target: if a previous fight did not end with mobs killed, they keep their rage bar loaded, and will discharge their rage attack quickly on the next character unfortunate to aggro them.
Typical examples of trash mobs for which this matters include bears, beetles, ogres. Bosses are expected to use their rage attack eventually, or even multiple times, but for those that stun/displace/disable it often means a bad start.
I haven't seen this discussed. Is it expected and intended? Desirable? I would have expected the rage bar to slowly return to zero after a fight ends.