This behaviour may just be down to the Severn - it does do some very odd things. For example, the Two Severn Kings can still be seen, and, more notably, the tide can be seen as a reflected wave travelling back from Maisemore Weir*. That is, you can still see the situation where the tide hasn't yet filled a part of the river, yet the bore travels downstream out of it, and another where the bore goes into an area and is then 'spewed' out of it.
Generally, the Severn is a lethal river. It has the second highest tidal range in the world, and the tide comes across the flats of the Gwent Levels (between Cardiff and Caldicot) at some 11 knots (~20.3 kmh-1)*
However, all this said, it seems equally possible, as will be explored here, that the Severn itself is only a bit player in this wonder.