My heroes just were downed by Sentry’s World Rupture skill (The Gauntlet event). It was fired in despite of Sentry stunned condition. Isn’t that a bug? 
Gauntlet is nice because it reduces the grind but there seems to be some bugs in it that’s for sure.
From not being able to use TUs if the AI is using that TU, to this one that you experienced to a Colossus issue that I noticed where is Fastball special still goes off even though he has been downed.
The bugs are minor though so I am willing to put up with it for a bit of a break from the regular PvE grind.
Did Hood reduce the countdowns to 0 with his Intimidation skill? That’s about the only legitimate way I can think that might happen. Otherwise, it sounds like a bug.
Hood was already downed at that moment.