This problem can arise as a result of the "wrong" kind of stuff.

If your work consists a lot of reactive tasks - say you work in a call centre or you have a customer service role - you will spend a lot of your time working to other peoples' agendas rather than your own. This then means that your time for clearing "In" is more limited and overwhelm is more likely to occur.

It's not just how much stuff comes your way but the type of stuff.