What would cause bitcoin to drop 30% or more?
There are many things that can cause Bitcoin price to fall massively. Most times it couldn't be rumors or speculation which causes a lot of people to sell and this will eventually cause Bitcoin price to dip drastically. Sometimes when exchange has been hacked and huge amount of coins are sold, the market will crash and even other people who left their coins on different exchange will sell and the sell oder will be higher than the buy oder making it dip. Infact anything that creat panic in the market causing people to sell more than buy, that will eventually make Bitcoin fall drastically.
However, such declines are not long-term, they recover over time in the long term. In fact, such a negative event causes Bitcoin to dump a little, but later Bitcoin pumps more strongly. That is why instead of getting scared when you see such a dump, you should think long-term. Such short-term dumps or corrections of Bitcoin often happen, but they always recover later. So buying more during such a decline will give you better returns later. So you should buy more during the dump.