OK, leaving aside the modernist anxieties around sex, race, and gender-appropriate behavior; reflecting that people in this region didn't get murderously tense about premarital or extramarital sex until sexually-transmitted diseases had done a number on Victorian-era peoples, a thousand years later; and that being sexually active was, to the contrary, an important social & survival characteristic, unlike now & unlike Northern Med & some (not all) Asian areas back then...
Securing the feet is a very old method of keeping a dead person's spirit in place, literally preventing the dead from walking. Burial face-down is traditionally meant to confuse the ghost so they can't find the living, based on the understanding that our eyes are on the front of our heads and the dead aren't flexible or clever.
These people *murdered and secured* this person (who was considered an adult at the time) and dismantled & abandoned their settlement. This is *so bad.* I mean, we fuss about having to pack our stuff up, but we don't have to take down our houses when we move!
There were so many horrible things going on at this point -- invasions, Viking raids (which were a lot more horrible and a lot less sexy than TV might make you think), drought, floods, and absolutely relentless famine; and Mercia was right in the thick of it -- that it looks like something horrible hit the village and they picked her as a scapegoat.
* Maybe she brought a virulent illness; plague existed in an early form then.
* Maybe she was already an outcast, and convenient to blame.
* Maybe she told raiders where to find her village in an outburst of adolescent angst, and reported back in a fit of remorse or was found out. Traitors are never popular at home.
* Maybe she was just a mean, ghastly person, seen as capable of great evil. Some people are!
Given the times and the situation, the villagers went to extreme lengths to remove themselves and secure that person to that spot after death. Something awful happened and they felt justified in laying the blame on her, before they took down the village and disappeared. I'd be fascinated to find out why.