Is it really due to constant versus triggered abilities? I thought the rule was in fact the opposite, that constant abilities have priority over conditional abilities?

Is it not that Imp “a card you access” relates to the period of accessing a card (and resolves when you would normally have the option to trash things within that period), rather than triggers such as “when you encounter” (or indeed “when you access”) which are resolved at the instant when the encounter (or access) period initialises?