There were two main theories I saw.
One was that mining rates dropped due to flooding in China which temporarily took out ~10-30% of the total hashrate.
I don't buy this theory.
The hashrate drop started somewhere Friday, just one day after the retargeting, it was visible on every graph
on Saturday and nearly all in the users knew about it the drop, it was pretty obvious, so why would the sell-off start 24h later and be just an instant crash with no prolonged repercussions?
The situation in China hasn't stabilized yet, the fees are through the roof, all the trigger elements in this scenario are still there and affecting the network yet the price is recovering even with no indicator that the entire hashrate isn't done for.
Although I don't like the manipulation theories at all and usually I try to toss them aside a huge sell to wipe out all those long positions by somebody who had a lot to gain from might be a better scenario. I don't think it's that simple, I don't like it but there is nothing else, all the other events are simply too far away from a timing perspective.
We never really know why the market is affected this way or that way, but the theory makes sense... The incident in China could had been just the trigger and what happened after (price dropping), the consequence. Investors see something worrying happened, so they tend to get out the market, causing a considerable drop on that moment, but once they see the situation isn't so serious (or that it's not going to become worse), the market starts recovering slowly again, although the trigger incident issue isn't totally solved yet.