I cleaned my cassette and changed my chain, now my freehub is slipping. What are the possible causes?
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.