So, what is it?
My limited research (googling - a verb now!) suggests it is a Windows Installer, connected to the Windows Update Service. Even when you are not updating, it may be tidying and compressing old update files in the background!
There are multiple links to problems that used to be present with tiworker.exe, all reportedly resolved by Windows Updates (Guess what I'm doing next...)
- A servicing stack update is available for Windows 8 and Windows Server 2012
- Windows 8 update bug clogs CPUs