If I had to guess, and going by the fact that subsequent runs resulted in you also hitting this rock, I would guess you're describing Target Fixation (
wikipedia).
About 4 or 5 years ago 3 friends and I were pinning it down Cressy in a very tight formation as you do when you're being stupid, and somewhere along the highest speed section at the bottom the guy in front of me hit a rut and got wobbly so I edged to to the left to avoid crashing into him. I misjudged speed/direction/ability and went straight into the branches of a tree that were very close to the trail back then, and still have the scars of that encounter.
In addition, the guy about 30cm behind me followed my line and crashed into me, falling to my right into the guy in front who'd slowed down. Then the guy up back crashed on top of those two.
It was, quite literally, a pile-up. Dust everywhere, one guy walking around winded and in shock, sucking down big ones. Me bleeding everywhere. The other two with some grazes. And eventually, all of us pissing ourselves laughing. It has become known as "the Cressy pile-up" since then.
Anyway ... for at least a year afterwards, literally every time I rode Cressy and tried to maintain speed in the same spot, I would crash into that same tree. Again. Again. Again. It literally felt like some kind of black hole, sucking me straight into it.
I finally figured out that I was looking directly at it each time thinking: "That fucking tree" as I headed towards it. I got to the bottom one day without hitting it, that I realised I forgot I'd even passed it, then consciously avoided looking at it next time, and hey presto. Problem gone.