"presumably flooded at times by the Kennet"
Well, it's certainly right against the flood plain so it could also be said to being prone to being periodically almost surrounded by water in a startlingly Silburyesque manner - depending if you're a believer or not.
"And why did the Romans "trash" it"
But did they? Where exactly was their road relative to the modern one? Maybe the actual road missed it and the only incursion was the road's built-up embankment ONTO it (and a lot less than in modern times)? That might be an important distinction to the Romans.
I have mentioned a couple of times I think its not round but has a number of distinct facets (like Silbury) but no-one has agreed or disagreed. I wish someone would go and look.
A statistical question: what are the chances of a broken (votive?) bronze age axe turning up at the base (indeed, in the "moat") of a mound that isn't Bronze Age?