As the flow from these reservoirs flows into the two leats that cross under Rhiwbach incline number three and then down towards Blaenau and normally contain a reasonabl flow I doubt the extra water would have been noticed.
It would certainly seem someone had planned this (especially as both reservoirs were targetted a while apart) quite carefully.
Someone must have a grudge either against the quarry, or RHD himself. I gather he is not the most popular man in Blaenau.