Many things. Escrow is a third party in California - and not a party to a transaction.  

The process: Open escrow. Review the Preliminary Title Report to ensure there are not ghosts on the title, ensure all the timelines are met, deposit funds are wired on time, parties sign off, and closing funds arrive on time.

I suggest you ask an escrow officer for a flow chart - even that will not cover the things that show up and must be dealt with.