Looking at it from a simplistic standpoint, the CEO holds a fair amount of Ethereum, more than Bitcoin even. It could be why Coinbase is doing things like adding ZRX which also runs on Ethereum. It could all be as simple as they have Ethereum and want to make more money on it.

The other theory I touched on yesterday on your other post. Still working on filling in details there though.

https://www.ccn.com/why-ceos-of-circle-coinbase-and-major-crypto-firms-are-bullish-on-ethereum/