One explanation is that he sat down and lost consciousness (due to heat exhaustion and hyperthermia) and died very soon after. Rigor Mortis set in and locked him to the position he died in.
Another possibility is that Ellen tried to prop him up after she found him passed out/dead and then rigor mortis would have set in and kept him in that position.
There's still no official update on approx. time of their deaths for whatever reason there may be.
They were found on tuesday morning, so that's about 36 hours after if they perished around or before 6pm on sunday.
On high temperature both internal and externally, I don't know how rigor mortis would have left stiffening effect even after 24 hours later unless they died much later than the aforementioned time.