What reasons might this be happening? I have a 1×10 setup with an x9 mech. The cassette is a bit worn but don’t know if this is the cause.
Usually falls off on a downhill section when the going gets a bit bumpy.

Check the B screw hasn’t been forced out….we had a demo bike today with an X 7 mech brand new, popped the screw clean through the threads of the mech! ❗

Does it slip in the front or rear? What chainring are you using? Chain guide?

The b screw is fine, I’ve tried screwing it in some to see if that makes a difference and I will test it tomorrow. There’s no chain guide at the moment, I haven’t got mounting points on the frame so I would have to get another fixing for that. It slips off the front cog which is a 32t

Are you using a narrow/wide chainring? You’d need one of those if you want to ride off road without a chain guide. There’s simply not enough retention to keep the chain on otherwise as standard chainrings are ramped for shifting and rely on the front mech to keep the chain on.

If you are already using a N/W chainring then you might want to look at chain length – you can probably trim it down slightly. Worst case scenario is using a chain guide but most seem to manage without them nowadays so keep that as a last resort (unless you really want one).

If you’re not using a clutch rear mech minimum you need upfront should be a NW chainring, a standard chain ring a chain will jump off as the mech swings back and forth on the descents and bumps it will cause slack in the chain wrapped around the chain ring hence it derailing.

Worn chain, then worn N/W were the cause of mine last year.

A narrow wide ring will help as would a clutch mech. It does depend on wear you ride though some riders get away without any of this on a 110mm setup I know I did when I ran 1×9 nearly 10 years ago as I wore out the out chainring bent the inner so removed them, along with the mech and shifter. East anglia is not bumpy though compared to some places.

Make sure the clutch is actually doing its job. You can take off the plastic cover and there is a torx fitting which you can rotate clockwise to add tension.