Yeah it is extremely strange. In some respects I can see how they wouldn't anticipate some of the demand for certain coins and there ends up being a weak point in their setup that gets way overloaded. The fact that there was like 23 Billion in volume for DOGE and that it passed Litecoin was unreal and really never seen before. I think once some of the exchanges had issues other things fell like dominoes. Especially when the quasi exchanges weren't able to purchase from real exchanges went down and then the Dogecoin network itself was experiencing extreme pressure and fees were going up.
Like I think what happened to HotBit was that their Dogecoin node experienced so many requests that it couldn't keep up and something failed and they tried to upgrade it and probably had to Resync with the blockchain so they had to shut off deposits and withdraws.
Something like Robinhood comes into question because we don't know how they manage funds on the back end or pretend to but probably what happens is when people are buying they are buying on the back end from real exchanges and once there were issues with that they had to limit people's buys but if someone sold that probably put that person's coins back in the pool for someone else on Robinhood to buy. This still doesn't excuse them for what they did with GameStop Stock because that is different.
Kraken went down, Voyager I guess went down as well, HotBit had to shut off withdraws and deposits. Bittrex had to shut off withdraws and deposits as well. Coinbase experienced outages even though they for some reason still don't have DOGE on their platform. The one that seem to stay up was Binance which has unreal volume.