Leaverbuster warning without leaving or afking at all?

I have been playing a few aram matches tonight and randomly at the end of one of my matches I received a warning from leaverbuster that if I afk in any more matches, action will be taken against me. Why? I dc'd in one match about 6 games back and have not afk'd or disconnected a single time in the matches leading up to this warning. Is it because sometimes I tab out after the nexus explodes and leave the client running without hitting accept? Maybe it's the fact that sometimes I alt f4 to close the client at the end of a match instead of pressing accept? In any case, I'm not afk ever and I'm afraid I'm gonna get banned just for how I interact with the client.