Pull on the mask cup, but don't worry about pulling too hard. The hose is quite long and you'd probably have to pull it down to your crotch to start putting any undue force on the hose (The hose also runs through a hole in a bracket which has a rubber grommet, which will offer a degree of resistance, so it would be quite difficult to pull the hose away from the oxygen generator).
You are absolutely right however you should still make sure you pull on the mask rather than the hose as you could still pull it out. Imagine in an act of panic...
Can't remember where I've seen it (it was one of those Nat Geo docs, for what they are worth.
)