Well it was a whole bunch of things - basically that he was cheating with a bunch of girls, that he was 'preying on fans' by taking money and gifts (like a sugar baby situation) and stringing them along and more alarming accusations of coercion. Additionally, lots of comments hating on his group mates and how they were taking job opportunities.
In China, this was also the time the Kris Wu case was breaking and while that was a much more serious situation, this was a time of serious concerns around behavior seen as predatory towards fans by kpop stars. SM usually doesn't comment on cheating scandals but went out of its way to not only apologize but ensure he posted a more specific "sorry for my behavior and those I hurt" apology, which certainly made it seem like there was smoke to the fire. (In contrast, on Chanyeol, for instance, SM didn't say shit.)
NCT's Lucas and SM Entertainment have released statements regarding the rumors of his actions towards his past girlfriends. On August 23, a Korean netizen
www.soompi.com
His fans went and did a bunch of their own investigations trying to prove it was a setup and I'm sure you can easily find that info, but again, SM pulled a fairly popular idol from everything soo...YMMV, make of it what you will.
Chinese fans felt very very strongly against him, as did Korean fans, so SM parked him, as they often do for scandals.
Now if SM is parking someone for a scandal, they usually bide their time, wait for things to cool off and then start testing the waters with appearances and stuff to gauge the temperature. Nope. Nada.
He posted some cryptic posts to his IG every time the group had a big moment without him (and not msgs of support, mind you), which would get his fanbase buzzing about him. (IMO, it seemed petty af and didn't really garner sympathy to his side.) SM is lazy as hell, but when they started photoshopping him out of pictures at events and re-recording the back tracks without him, it was pretty clear he wasn't coming back (which again, smoke, etc.)
When Chris Lee, the CEO, posted a pic with him, there was a very very strong backlash from K- and C-fans, which maybe finally sealed the deal? Dunno. Fundamentally, NCT is the company's cash cow and the bulk of their sales and they don't want to do anything to risk that. When K-Czennies boycotted Superhuman, it was impactful - and the group wasn't nearly as popular then.