Apparently Cass ran into a pair of fans, who are gay men, Matthew and Mondee, and during their interaction Cassper slipped a "no homo" in there while speaking to Matthew. Many may not know why that would be considered offensive because of it's prevalence in society, especially Hip Hop.
"There was something in the picture, when we took the picture and he said he 'wanted in there'. I was joking when I said, 'Hey, no homo, though'. I think he took offense."
The reasoning behind it is fairly simple. Firstly, that phrase is usually used to further segregate the LGBT community by straight people, when they not only allude to homosexuality in everyday interactions (e.g when complimenting each other) but state that it is not a thing they cosign as well. "No homo" feeds into homophobia by firstly painting it as a deviation then in a sense condemning it.
This in turn started a chain reaction and people reached back into the archives to look for further instances where Cassper perpetuated or performed homophobia, and of course, they found them. The tweets in questions have since been deleted from his main account, but well.
Cassper's side is that Matthew was "very drunk" during their interaction and he just said it as a passing joke. He then went on to share how he's performed at Pride and it was for free so he's surely an ally and not a PR angle.
According to TimesLive the man in question, Matthew Whitehead, has accepted the apology and says he hopes this will be a learning curve for the star.