There is a couple reasons for this change I can think of:
- On the Hunt stacking with effects like Righteous Glory or Shurelia's Equivilent and causing teams to run over enemy teams too quickly.
- On the Hunt's initiation is viewed as too strong; 6 seconds of 60% movement speed may be overbearing in teamfights later in the game when bruiser assassins like Rengar get access to more mobility to delete a single target with no reaction from the enemy team.
So the nerf isn't completely random. With the new addition of Glory, an item that seems to be fairly popular, it was only a matter of time before Riot looked into toning down the effect of having multiple team-wide speed buffs. Unfortunately this means Sivir loses power for an item she doesn't even build, but has her allies build (supports and junglers primarily.)
It's not an ultimate-destroying nerf, that would be if they touched the movement speed bonus, but it is significant enough to lower Sivir's level 11 and onward chasing potential.
TL:DR
Sivir gets along with team wide MS enhancing items. Got her chasing power later in the game dialed back for it.