It looks a lot like there's a power station just downstream (photo), which can be seen on openstreetmap. I suspect that the power plant is being used to respond to peak loads for which hydro is very good. That whole stretch of river has several power stations and the flow through them will be coordinated to some extent. In particular there's a dam upstream of the falls.
TV pickup is a phenomenon where electricity demand spikes in the advertising breaks of popular TV programmes. It may be more of an issue in the UK than in some countries because of of the national obsession with tea-drinking and a significant fraction of the popoulation putting the kettle on at the same time but can account for surges in demand on the timescale you mention. It's also possible tht related to this you happened to be there at the right time for some sort of test/maintenance operation which resulted in opening/closing gates.
I'm slightly surprised that there aren't any warning signs telling people to stay out of the water as levels may change without warning. I've seen those before downstream of power stations.