From an engineering perspective, I have to think the tires are made to operate at some ideal temp, and that they shed (or retain) heat in a more exponential manner when away from that ideal. Just a guess though, since I know little about rubber's heat coeffiicient and the effects of layering in the processes.
Yeah, hotter would be stickier to a point, but the speed rating for a tire, I believe, is there because of the temps that are attendant at high speeds. The higher temps probably cause the layers to separate from belts and each other, thus unraveling into a nasty situation.
But just some guesses
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