I suspected as much, but figured it wasn't my place to say so... honestly, why do people do this sort of thing? What do they gain out of it? 
Perhaps other mixers desperate to destroy the competition? There are some with a dodgy reputation who are probably struggling to lure victims. Then you have those offering genuine and legitimate mixing services but can't make ends meet with the low turnover from the few users mixing coins on their sites.
Then of course you have those malicious actors who take pleasure in destroying other people's work or who resent their success.
Envy knows no bounds. People filled with it will do things that cross the bounds of human decency.
Eh, it's actually fairly worthless to try to do it that way. A DDoS attack is only a temporary method of attacking a website in the sense that once it is over, it's over without any real consequences afterwards. The denial of service also can't really last that long without too many resources. I don't see any mixers really benefiting from doing this either, but it is certainly possible to try to get the business of those people trying to mix their coins at that time and then looking for an alternative. There really isn't that much to gain, though.
They might be looking for a large ransom, or it could be state actors. DDoS can last for a long period of time. There was a pretty long (IIRC over a week) DDoS on ProtonMail a while back that was very strong.