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.
This is the number one reason I think why that flash crash happened. I am on pc that time watching the 15mins timeframe of bitcoin. It's on $56k that time and in just a few minutes it went down to $50k. If this such events can happen then can they do it again in the future? What resultes for that flooding, why they are not able to prevent it.
The second reason was due to a funding rate indicator on leveraged positions that said there were a lot of longs open and a high funding rate for shorts (meaning short term shorts were paying people to open them and leave them open). I assume that could then build into a bias if the market starts to get a bit stagnant.
This is reasonable but I don't think that can completely cause that flash crash. We could just dump for couple of days so it is a healty correction.