I've seen this chart, which seems to explain why. But unfortunately I've lost the original reference - I'd be happy to be pointed at it.

In essence, there's an expectation that your workload will be lighter in the future than it is right now but this is never bourne out. The lesson is to say yes to something only if you'd be happy to do it next week, rather than assume you'd be free in 6 months time when you expect to do it.