Okay, I will give you my side of things. Not going to say it is true for all paladins, but here goes
In places like Karazhan where there are undead, we paladins have an even better aggro-gain spell, Holy Wrath. This spell has a 2 second cast though, and is tough to cast when you have multiple mobs beating on you. Ideally you want the holy wrath to go off just as the mobs are about to melee-hit you. So given it takes two seconds to cast, and consecrate triggers the 1 second global cooldown, I will try to lay down consecrate about 3 seconds before the mobs get into melee range.
The reason you might see mobs like undead that pass the paladin and suddenly turn back to him/her is that holy wrath is a high damage aoe spell. What might confuse you is that a paladin casting holy wrath looks very similar to casting a heal.
The other reason I might lay down consecrate is if we have a puller who has to go out front to pull then run back, I am hoping they see the consecrate and try to pull mobs over it. Consecrate is a relative low damage spell, so when mobs enter it I am hoping it will tick off a couple times by the time they reach melee range. If you cast it right when the mobs are on top of you, the first tick is not enough to keep an over-eager mage from pulling aggro almost instantly, making the paladin have to run all over the place to regrab aggro.
Those are my reasons, anyhow
