This used to happen to me on Luigi Circuit when I was powersliding fiercely without straightening my kart every now and then. I think of it as filling a buffer, and straightening out/letting go of R usually prevents it from filling up too quickly/resets the increment effect of the preceding slide. This is not fact, just how I look at it.
As I'm sure you know, you spin out after circle-sliding for too long, and you gradually build up towards the same situation when you're so frequently driving in the sliding state.

Andreas

Yes, as far as I know, at least.
I'm sure containing all the power that triggers a boost while doing another one directly after helps the factor greatly, but I'm very confident it's not specific to an arbitrary situation. It happens for a reason unrelated to the actual spot. It's all in what you do, not where you do it.

Andreas

I sometimes spin out on Retro Mario Circuit 2 rounding those two turns before the jump. Straightening out my kart before the next turn works for me as well.