A couple of guesses:
1) This happens to me once in a while this time of year because of increased package traffic.
2) UPS and Fedex are both raising rates and changing the way they charge for packages, prioritizing size over weight. I read that this has caused Amazon to explore options to reduce shipping cost. I think I ran into this over Christmas. I ordered one of those big 24qt Rubbermaid containers people use for camping, etc. for my Niece's husband. I was surprised to see that it was going to be delivered by Pilot, which for those not familiar is one of those local delivery services that you usually get stuff like big TVs, appliances, etc delivered by, curbside to your home, and they make a delivery appointment with you.
I was surprised by this because of how light the container was (maybe 5lbs). When it arrived, it was in a ginormous 5'x3' box. The container itself was only maybe 30"x15". One of the reasons I read for the carrier change in pricing is because they are more concerned with space, and merchants waste it with inefficient packaging. I could see their point with this particular package (and many others I have received that were smaller, but still had packaging 300% or more larger than the item).
Amazon sent me a survey after the delivery asking how I liked the service, so it looks like among other things, they are exploring new shippers. Also I see they are really pushing the "not in a hurry for delivery, here's $1 off on Amazon media" thing.