I keep my bitcoins on a trezor hardware wallet...if a hardfork occurs...will I still get 100 BTU ? So if I send 20 bitcoin to coinbase after the hardfork I will be credited with 20 bitcoin unlimited in BTU wallet ?
If you hold bitcoins on a trezor, then you hold the private keys to your bitcoins. Those private keys are what you need to control your bitcoins. After a hard fork, those same keys will work on both chains. So yes, if a HF occurs then you will have 100 coins on the BTU chain as well as 100 coins on the main chain.
You may need to educate yourself about replay attacks though. If you broadcast a transaction with the intent of moving your coins on one chain but not the other, a malicious actor could rebroadcast the transaction to the other network in such a manner that it moves your coins on the other chain too.