According to the Encyclopedia Britannica article on Immanuel Kant, in the section discussing the Critique of pure reason:
In the Transcendental Analytic, the most crucial as well as the most difficult part of the book, he maintained that physics is a priori and synthetic because in its ordering of experience it uses concepts of a special sort. These concepts—“categories,” he called them—are not so much read out of experience as read into it and, hence, are a priori, or pure, as opposed to empirical. But they differ from empirical concepts in something more than their origin: their whole role in knowledge is different. For, whereas empirical concepts serve to correlate particular experiences and so to bring out in a detailed way how experience is ordered, the categories have the function of prescribing the general form that this detailed order must take. They belong, as it were, to the very framework of knowledge. But although they are indispensable for objective knowledge, the sole knowledge that the categories can yield is of objects of possible experience; they yield valid and real knowledge only when they are ordering what is given through sense in space and time.
This view that physics provides the general form of this detailed order seems to be incorrect because physics is now done differently. Before, physics was the development of equations to predict events (classical mechanics). Now, physics is the development of equations to determine probabilities of events (quantum mechanics). Furthermore, spacetime is no longer considered static. So how can physics be a priori then?