The question "How do you know he is deliberately leaving the towel on the floor?" is a good one. Unless he leaves it neatly folded there, you cannot really tell.
I suspect it might be deliberate, however, for the sake of what we sometimes call in my part of England 'buggerance'. What this means is that the man's wife gets irritated because the husband drops the towel on the floor. The husband is probably being told he is doing a thousand different things that are wrong - like putting the newspaper in the wrong place when he has finished with it, not taking his coffee cup to the kitchen etc etc.
At some point he subconsciously and irrationally decides "Enough". Right or wrong, he feels emasculated by these constant reprimands - so as a form of passive resistance, he starts to do things that are deliberately annoying.
I suspect this happens often in houses where couples try to avoid a face to face row, and the tensions build up and this antagonism replaces communication.
The towel on the floor might, and I say might, be a symptom of non communication. The husband has not even consciously thought about it, but the action will almost certainly be driven by emotion. The emotion, I guess is frustration and disappointment that the relationship has, in his eyes, degenerated to one of domestic familiarity where communication is limited to the wife enumerating his faults.
I'd suggest that the wife might try looking at the overall atmosphere in the house, how much the couple actually talks to each other and what they talk about and how much positive communication there is.