We had a lesson about Pompeii today in class and one of my pupils pointed out that on some of the pictures we saw from the dead bodies the people seem to have been rather in movement. For example one man was laying but his upper body was rised and he leaned on his arms like he was crawling away. Neither them nor I could find an explanation for those positions since we both agreed that usually when you die after a while you would sink to the ground when the muscels in your body relax during the first hours after death.
It is the result of the heat of the pyroclastic flow that killed them. Think of a steak when you put it in a searing hot pan, it contracts. Similarly the muscles of the victims of Vesuvius contracted in death as the super heated gas and fine particles of the pyroclastic flow engulfed them. As the flow cooled, it formed those famous casts of the way their bodies contorted.
They were covered in ash and other volcanic materials (called pyroclastic flow) which fell at such a rate that it quickly encompassed them. This solidified, keeping them in the positions we see today. The whole eruption--from initial blast to the whole town being buried--happened within 24 hours.
That's the short answer.