Is it because I put a 11-36 on ?

Ellsworth say there hangers are good up to 34, didn’t really think about it?

Or is it more like the meck was a bit lose and bounced into the spokes?

Is anyone else running a big cassette and getting away with it?

If you chain was sized and fitted based on a 11-34 then fitting a 11-36 will need more chain length at the extreme big to big ratio. This, and maybe so extra growth requirements due to suspension, could have been the cause.

Was a new chain and was on the granny.

Apart from chain tension is there any other reason for not fitting a big casstte? Mech seems to reach with no issue.

is it a cludge mech by any chance?

No its an old sram x9