I think this might be there to give Nunu more opportunities to have an impact in a fight, although how it's being done isn't really all that interesting.
A lot of the changes to tanks recently have been a lot about giving them more spammable abilities or persistent effects that give the champion less periods of downtime in a fight and more opportunities for them to contribute in a fight. Leona's recent changes to her Q and R are a good example of this kind of change, while the cooldown reduction to Nautilus' E last year is also a good example of this kind of change, and while it wouldn't seem like it I think Riot wants Nunu to have a W that has more impact in teamfights.
Having his W at a lower cooldown lets him use it a lot more if he survives in a teamfight, while the duration nerf is necessary to compensate for that change. It's also harder to have 100% uptime on it because you need more cdr (15 second CD with 12 second duration needs only 20% cdr for 100% uptime, while 10 second cd with 7 second duration means you need 30% cdr, which you already get on your Nunu build anyways). Mana cost is reduced to account for more usage as well.
The movement speed change is interesting. It's now a flat 10% at all ranks, meaning that the only incentive to level it up is to get extra attack speed. This leaves it as a better 1-point wonder in a world where Nunu already uses it as a 1-point wonder, especially when you don't have any carries that want extra attack speed.