Front tyre worn before the rear?

[Phil TK]

This sounds like the phenomena known as 'cupping'. It's been discussed many times on various forums and certain tyres and bikes and/or combinations of seem to be prone to it.
Unfortunately, as far as I know, nobody really knows why it happens and it would be difficult to prove any theory.
When it happens, just changing the tyres for another make seems to be the proper course of action.