said by Rickkins:
I think I located the issue. My kid has been playing online games with 'steam', which from what I hear tends to hog yer bandwidth.
Playing games takes very little bandwidth. Downloading games is the only way you'd notice something and that should be pretty rare unless you guys are dropping thousands of dollars weekly on Steam games.
but I have to say that since the upload rate is roughly constant, a steam game download seems like a plausible cause.