Basically since Warwick was founded, it had a reputation for being the place people went to when they were rejected from oxbridge. This sounds bad but one subtlety with the U.K. system is that almost all students may apply to at most one of Oxford and Cambridge in any given year and those universities tend to get more sufficiently good applicants than they take in. If one of Oxford or Cambridge is commonly considered better than the other by applicants then Warwick the distribution of the abilities of the rejects is higher than if they had all applied to a single top university equal in size to the sum of Oxford and Cambridge. Therefore Warwick gets to take in lots of very good undergraduates.

The university was founded in the 60s out of what used to be a research retreat for mathematicians. At its inception a large portion of its mathematics department (at least the topologies) came directly from Cambridge (I believe the story is that Christopher Zeeman went around his colleagues telling them that all the others were going to Warwick). And in the mathematics department in particular has been strong ever since.

I believe the course structure is also different there than from oxbridge (or many other U.K. universities) in that it is meant to be more like an American university with more choice to pursue courses from different subjects or with wide varieties of specialisation.