Was it a pre-made 5? I am 99.9999% certain the Doom Bots didnt reported you.
First off, you would never be banned if no one reported you....so you got reported by SOMEONE (presumably one of the other 4 human players and not the bots).
If it was a premade 5, then you need better background check on your "friends" and probably get better friends because one of them most definitely reported you..
If it was not a premade 5, then someone did not appreciate you used one of the zero tolerance word and reported you. Remember that even though you did not direct the word at any human in particular, you still subjected that word on your other 4 teammates who had to read it and their game might be ruined by it.
I personally would not care enough to report you if you directed it at the bots (those cheating bastards kinda deserved it), but it can and probably did offended someone with its usage.