Step deck trailers like that use what we refer to as donut tires, 15" or 17.5" diameter to allow for the low loading height, increased interior height to be able to stack cars on the baskets in the trailer with additional clearance.
Because they turn at higher RPMs than the normal 22.5" or 24" tires on most semi trailers, keeping them inflated to spec, 120 psi is critical to prevent heat build up. It is well known operational problem in the industry, has been for years.
the routine inflation checks were likely not done, the smaller rims make it very difficult to check the inner tire for proper inflation. there are systems out there to make it easier, but they are widely ignored.
Unless it went flat very quickly, which is possible between routine walk around checks during stops, probably bad maintenance.