It does sound funny to me, so I'll explain why, but I think this usage might be unremarkable to other native speakers.
When I hear slammed into, I think of a momentary action with no duration, but in this example, the verb appears to be describing a continuous state that was entered into when the slamming occurred. That is, it sounds like they were slammed into the ground (which is fine), but then stayed slammed into the ground (which is odd, at least to me).
I reviewed the first 100 results for slammed into on COCA, and all of them were used in the way I expected (as a momentary action, not as a change of state). Although that doesn't mean this usage is wrong, it does provide some support for the idea that this isn't how slammed into is usually used.
My conclusion? It does sound funny to me, but my feeling isn't strong enough to call it "wrong".