there's no movement during upkeep so no trap trigger!
Unless returning from a Banish triggers traps. This is somewhat doubtful, but has been ruled proper play by Shad0w.
More importantly, it's an example of why you should never assume something can't happen. There are no attacks during movement (except when there are). There is no opportunity to reveal an enchantment during the reveal of another enchantment (except when there is). There's nothing you can do with an incapacitated creature (except that there was), etc.
Murphy's Law says, "If something can go wrong it will go wrong." This is often taken as a pithy expression of pessimism and the perversity of fate, but it's also a very useful design principle: the example I always think of is that if there's two ways to connect a plug to a socket, and one of those ways is wrong (upside down), eventually someone will do it. So, to combat this is, we use polarized plugs that can only fit one way. By making the failure mode impossible we prevent the failure. Conversely, we have to assume that everything that we haven't directly made impossible will happen, sooner or later.
So, for example, if a curse were printed that caused a creature to teleport randomly in the Upkeep, that would count as movement, and so would trigger traps. Moving isn't forbidden in the Upkeep, so it's sensible to assume that can happen, even if you don't immediately see how.
