There is some body of work on why surprise attacks work so well so often, which suggests that there is, psychologically, a very substantial barrier between not-fighting and fighting.
It is usually examined at the level of the individual. But I expect that it also accumulates in military hierarchies and societies as a whole.
An individual who is already adapted to fighting, is already emotionally and rationally on the fighting side of the divide, has a great advantage over an individual who is on the not-fighting side emotionally and rationally.
None of the actions of the fighting individual will fit into normal action-reaction paradigms expected by those who are not fighting, and so the not-fighting individual won't know how to react to actions already witnessed, nor how to anticipate the next actions of the fighting individual. Until the mindset is shifted, the not-fighting individual will be stuck in a sort of paralysis, trying to gather enough information to make sense of what's going on, rather than acting on what has already been observed.
The scenes in Fury where the new guy doesn't shoot his gun because he doesn't understand what's going on around him, and just kind of gapes at the noise and flashing lights rather than participating, is actually a reasonable illustration of the phenomenon.
It's not only contributes to the success of surprise attacks, but also to veteran formations vs. green formations, and to rapidly escalating the violence of a response beyond proportion to the original stimulus -- in any case you have one side that already has a clear understanding that the fighting paradigm is what's going on, the other other that doesn't start with that understanding, and has to get to that understanding before any useful decisions can be made.
Looking at Singapore in particular, there seems to be an institutional version of this going on. Percival and the British hierarchy just didn't have a wartime mindset. Maybe a policing the colony mindset, maybe a "they'd never dare to try" mindset, but not an "oh sh!t we better be ready for a real fight" way of thinking.
The Japanese in 1941 had already been actively at war for several years. They were fully acclimated to the "oh sh!t we better be ready for a real fight" way of thinking. That provides a very significant advantage.
At least that's my read of it all…
-Mark
(aka: Mk 1)