But I don't think anyone suggests that as a market reaction is obviously just that, a real time reaction. It might turn out that the market has missed some subtleties or there may be some new, or previously misunderstood effects here but broadly speaking this budget looks like it will be bad for the UK economy and the markets are just reflecting that. I.e. the markets are not saying the budget is definitely wrong, or that it is completely wrong only that on balance and in the aggregate it is very likely wrong in significant part and the risk of bad things happening has increased.
Also don't forget most of this was already priced in as it has been expected for some time. So a better question is, I think, what are they reacting to? I think the answer is probably the unexpected scale of the tax cuts and the scrapping of the 45% band. I.e. the market is not saying this is definitely bad, but rather that the bits we already expected to be bad were done on a significantly larger scale than we had been expecting. To paraphrase a guitar term "Louder is more bad"
Although of course the market reaction is decisive in that it has actually just happened and there are real world effects that materially affect our nation's wealth.