In case you didn't know, AlphaBay, the largest darknet market, went offline a few days back. Many believe it is an exit scam.
The resulting panic and chaos, however, is not the cause for the crashage.
The alphabay admins are pretty damn smart. They planned this exit scam three months in advance, slowly siphoning all of the market's cold wallet funds out of BTC/XMR and into pretty much every coin with a reasonably liquid exchange. This was the cause of the strong and steady climb in price across pretty much the entire crypto space -- except for BTC -- that started in mid May.
Now that they have pulled the plug, they are unwinding those positions.
Perpetrating their heist in BTC would leave them vulnerable to chain analysis, and the amount they've made off with is so damn huge that exchanging it out of XMR to BTC would have blown their cover -- XMR may have bulletproof anonymity, but if they didn't want to hold their loot in the form of XMR until retirement they'd get caught eventually moving such a large amount through an exchange.
So instead, they basically used the entire cryptocurrency space as one enormous mixer. Split up the loot across 200-some altcoins, make sure you cash out to separate wallets (one or more per coin) and never comingle the proceeds, and you have a reasonable chance of getting away.
So yeah. It sucks. (and then there's the whole stupid ICO thing, great camouflage
Further insight here which I think corroborates, although I would argue that the fee spike was worsened by the fact that the cold wallets were getting drained to exchanges in a massive number of tiny transactions. And when you're stealing the money fees don't seem to matter as much.
The fact that BTC is crashing along with the alts leads me to conclude that they are not just dumping their RandomCoin positions for BTC, but also turning around and dumping that BTC for fiat or goods. Perhaps they foresaw that their stunt -- and the brief though jarring loss of the main source of demand for crypto -- would crash the entire crypto space.