What caused this Blowout on inner rear dual bus tire

I've uploaded a picture of the INNER dual tire blowout. It was a city bus. It came out of the shop 3700 miles prior, less than a month. The mechanic measured the tread depths RRI = 12/32 RRO = 17/32. When blowout happened the RRO was measured at 9/32. The company fired the mechanic stating he falsified the tread depth and the tire should been replaced. Is it possible that if the inner dual was low psi could it have worn to that degree and blowout? Any ideas please.

 

The inner tire is the one that blew AND it was the one with the lower tread 12/32. I'm presuming the inner tire carries more weight, however the outer tire (17/32) had to of been low psi for the inner to be overworked?

 

Actually all if his tire measurements went up from what a previous mechanic wrote down 6000 miles prior. That tire was at 8/32 6000 miles prior. The vehicle was in shop prior but no record of tire change from other mechs. They take the reading in teams of 2. One measures one writes down. True the last 4 drivers were disciplined for not performing pm. However. That bus was driven by 10 different drivers that month. I'm arguing that since the first 6 drivers didn't annotate worn inner tire, then his measurements had to be good.