Has the Bitcoin crash just started?
In my opinion, no. Obviously bitcoin has stopped its rise to $20k but that doesn't mean that it is not able to break it again some time in the future. In fact i believe that $20k is going to fall to bitcoin by the end of the year.
These sort of dips are usual, especially if you take into account the rise of alts in comparison to bitcoin.
When alts rise, usually bitcoin will go down and that is exactly what's happening. You're seeing bitcoin dominance go down under 50% which is highly artificially low. IT'll come up to 50%+ once again very soon, and bitcoin will rise while alts hype dies off.