Beats me, maybe somebody visited the grave, heard noises inside and proceeded to move the stone. (BTW, thanks for the link.)
My understanding is that while some were crucified with nails driven through the arms, others may have been tied to the cross with ropes. (Death typically came via asphyxiation.) A weakened man with bones intact might be able to move a heavy stone.
As Jesus was not a particularly high-profile criminal, we shouldn’t necessarily conclude that he would be given unusually harsh treatment. If Jesus was perceived to be such a big threat to the Roman state, it seems likely that at least some of his followers would have been immediately hunted down. Given Christianity’s fondness for naming martyrs, it seems safe to say that if any followers of Jesus were subsequently rounded up by Roman authorities, that we would have reports of it.