Hear me out
Who is Foxy9's idol? Tenz
Where did Tenz play at the start of his career? C9
What if Foxy9 was a C9 fan and wanted them to retain Tenz but when they let him go and he instantly won Masters, he got mad pissed about the team and he kept his name "FuckC9"
Few years later, when he slowly began coming up to a semi professional level, it became clear that he can't keep the "FuckC9" name and he found an alternative in Foxy9 which also sounds like "FuckC9" in a weird accent.
In conclusion, he is just a hopeless C9 fans who is pissed they let his idol go and have been ass since then.