I am not an expert by any means here. However, I've never seen SR go down just because someone hits your rear bumper. The other way around, sure - you touch someone's bumper, it may or may not affect SR. But usually not if someone touches you. I suppose it's possible if you did something like slam on your brakes, but other than that, I'm dubious.
There's a lot of reason SR can go down. Last week (I think it was, anyway), for example, going off track stopped you from getting a CRB, even if you didn't get a penalty. I don't know if it affected SR or not, but a couple of times I didn't get CRB and had to look back through the replay to see that I did, in fact, go slightly off track. So it's easy to forget that something happened that might affect your SR.
This week, touching some of the kerbs doesn't give you a penalty, but there's no guarantee it doesn't affect SR.
And by the way, I got bump-drafted a handful of times in a couple of races at TM yesterday. I still got CRB, and my SR went up, although I was close to full anyway, so it wasn't by much.