tl;dr: Delay happened after cryogenic fuel load and before ignition, suggesting it might be an issue with dynamic loads caused by thermal contraction in the interface between the rocket and the pad. But this is very speculative; SpaceX themselves haven't said anything, some issues are expected when you do a wet dress rehearsal for a new rocket type that's never had one, and there are a bajillion other possibilities.
It has also been unusually cold in Florida recently.
If they blow it, they’ll come close to the Soviet N1 explosion.