I don't use coinbase, but Xapo does exactly what you describe. They instantly transfer the funds to another wallet and move them around. Probably some algos deciding what goes into cold storage and what stays, based on current outgoing flow demand.
At the same time that you're sending btc to coinbase, many users will be withdrawing; hence they probably have developed attempts to optimize this flow of funds in the blockchain--while still retaining a number of transactions off-hot wallets.