Yeah, actually i'm nott so surprised. Trees has a tendency to seek out the nourishment they can find. And a dead corpse add nourishment to the soil with the result that trees can often be found on top of graves. In an earlier era people where buried under stone mounds so the roots could not reach the body but with christianity it often was a treecoffin which moulds away rather quickly leaving free access to treeroots. If this was a single grave or just a part of a gravefield far to early to say but he certainly looks placed, that is buried, in the proper midieval position which makes me doubt a battlefield thoery. The first signs points towards a violent death but that is not unusual. England-ireland was rather restless during the time.