Just a weird issue with those circling birds; they'll be able to attack players/followers/minions without actually coming down from the air sometimes.
Here's what happened: after fighting a bunch of enemies, the circling birds that were trailing me came down from the air (as is normal), but either latency or some sort of glitch locked one of the birds into an attack loop on me and my Gargantuan. This was after the enemies had all been killed; I'm not sure if I had killed this bird already as I was focusing on more important monsters. Both my Gargantuan and I were taking damage from this bird, but the Gargan refused to attack the bird and I couldn't actually land a hit.
Also worth noting that the bird's attack loop wasn't grounded. It seemed like the initial swooping animation; the bird was in the air one moment, then swooping in to attack and then back again. Hope that helps narrow things down.
Returning to the Hidden Camp and reentering the Oasis via Waypoint fixed the problem -- I could kill the bird after I came back -- but I thought I would report this anyway.