I cleaned my cassette and changed my chain, now my freehub is slipping. What are the possible causes? I removed my cassette, cleaned it and put a new chain on my mountain bike. All seemed to go well and went back together nicely. However when I took it out for a ride the free hub seems to be slipping with a sort of judder. It's like it engages, but then slips almost like the chain is slipping.

I'm fairly confident it's not the chain slipping though as it's doing it an every gear (more so in lower gears where there is more tension) and the derailleur doesn't seem to bounce/move at all when it slips.

I pulled the cassette back off to check it all looked like it's assembled correctly, but I see nothing wrong. 

Thanks!

Update 1
I've inspected the cassette fairly thoroughly and am fairly convinced it's not warn. There is no angling on the teeth and no visible sign of the chain not sitting properly. Watching it carefully and it looks like the cassette is spinning with it when it slips, not the chain slipping over it. 

The old chain wasn't massively overused, it'd been on the bike maybe 8 months and done maybe 1000km. It was replaced due to a link failing, not because it'd warn out.

It's slipping a lot almost to the point it's completely unrideable. I'll stick the old chain on later and see if it stops, but I can't see such a massive difference being likely?

Update2
Thanks for all the answers trying to help. I've added some photos of the cassette. https://imgur.com/a/TY1Z7
It is likely that your cassette is worn. 

A new chain on a worn cassette will skip, because of the mismatch of the shape of the teeth on the cassette with the chain's spacing (which is shorter than what the cassette expects due to wear). 

You can visually inspect your cassette for wear by either looking for gaps in how the chain meshes with the cassette, or looking at the teeth to see that they aren't well shaped.