...
Contamination: molecules sticking to walls intermittently
main culprits
...
Water - from moisture in the air
...
Hydrocarbons (oils, plastics)
you know what's really oily? You.
...
detailed discussion the see pages on baking
Leaks: always happening, hopefully tiny
all vacuum chambers leak
really big leaks can be heard, usually from something like a mechanical connection that wasn't tightened
...