Killer Vent or stress from sudden tank change? (added pics)

Here's the story. I recently completed a rack for my frogs that I thought would better help me keep track of my frog’s behavior. Before this rack I had tanks scattered all over (in my room, basement, etc.)

Anyway, I have had this trio of vents since the fall (making them a little over 6 months old); they have been kept inside those plastic boxes with the colored tops. The boxes had close to 100% humidity, golden pothos, and sphagnum moss bottom with oak leaves. I fed them very often and they seemed to be very active.

Now since I finished the rack with 4 vertical 10 gallon tanks, I figured I would reserve one of the tanks for the vents, so I put them in. After being in there for 5 days I went to go feed them and noticed one of them slouched over a branch flattened. After taking him out I scanned the rest of the tank and found another inside of a deli cup dead. This one was the biggest frog of the three. The third vent was just hanging out on top of a piece of driftwood. I’m looking at the third now and it looks to be healthy and plump.

I have read on Saurian.net about the sudden movement of thumbs can cause stress that can ultimately kill them. I'm thinking this may have been the cause.

When I first put them in the new tank they mostly stood on the ground in the leaf litter. Where in the critter keeper they always stood at the top of the pothos.

Could the third frog have pounced on the other two frogs?
This really sucks... I will post pictures of the scene of the crime tomorrow.
Any input on the situation would be appreciated.
Regards
Sal

P.S Hopefully someone will read this post and prevent something like this from happening to them.