From the patch notes:
Thor - This unit will now prioritize attacking ground combat units over Medivacs.
Marines/Marauder are more dangerous for Thor. If medivacs has higher priority but you want Thor to hunt marines/marauders - you need to order Thor to attack marines directly. As there are usually a lot of them you will need to do a lot of junk-work.
After priority changing Thor will attack infantry himself. But if you want him to shoot down the medivacs - you will need to give him only few orders, usually medivacs quantity is much less then marines :)
That is my guess, though
EDIT: good way against Medivacs is Ghost's EMP - it will remove their energy and you can don't hunt them at all (at least right now).
So, I don't know why Blizzard felt the need to change it, but I can speak to how this will affect game play:
When you engage a MMM ball your Thors will no longer waste shots on Medivacs when there are higher priority targets.
I can understand how people consider Medivacs a real nuisance (I had to play someone today who took a page out of Select's book), but with a very high damage unit like a Thor which one shots Marines, having him target the Medivac is actually pulling dps away from vital targets. I realize you might consider the value of the Medivac in terms of what it heals, but they are notoriously hard to kill (even with the Thor's impressive range) and having the Thor prioritize it only leads to your opponent spreading out the damage more. While a Medivac can't heal itself it does move fast enough (not to mention having 1 armor) that it can get away. Meanwhile those high DPS Marines (or Marauders) are chipping away at your otherwise clumsy Thor.
Now for drops I can see how this priority scheme is annoying, but I think given the smaller scope of drops the micro required to make the Thor target the Medivac is small in comparison to getting the Thor to take down Marine/Marauders and having to retarget after every one.
Even for high level players removing that cost in APM will really benefit more interesting micro (just look at Fauxer).