I don't know... my 18 month old started with the whys about two months ago. I was sure he was just saying the word without undersatnding it, (especially because he's not verbal enough to say "why X" -- it's just plain "why" -- or actually "vy") but lately I've come to believe he really does understand. A typical conversation will go something like this:
Me: DS, don't climb on the table!
DS: Sorry (climbs back up again)
Me: DS, you can't climb on the table.
DS: Sorry! (like hello, I said I was sorry, leave me alone now)
Me: DS, saying "sorry" while you're climbing on the table doesn't make it better
Me: Because you have to stop doing it first.
DS: Vy? Vy? Table! Table! (kicking and screaming and I take him away from the table)\
He askes why at least 5-10 times a day, mostly when I tell him he can't do something, but sometimes just for clarification.
DS: That is? (what he says when he points at something whose name he doesn't know)
Me: That's a pair of tongs, we use it to take hot food out of the pot.
Me: So we don't hurt ourselves.
Me: Because hot water can burn.
DS: (muttering ot himself) tongs. Tongs.
ETA: My 31 MO also asks why ALL THE TIME, probably starting from about 20-22 months. That probably influenced when the baby started.