its because arrow speed was reduced. Lower ammunition speed= greatly increased speed bonus.
I'm having a hard time explaining it, so let me use this example
If an arrow travels at 100 m/s and a horse is running at it at 15 m/s, the arrow gets a 15% speed bonus.
If an arrow travels at 60 m/s and a horse is running at it at 15 m/s, the arrow now gets a 25% speed bonus.
The actual speed bonus I listed are by no means exact, and I'm sure a dev could correct them, but thats the concept of why horses are dying faster. This is also why throwing has always been the bane of all horsemen.
The change to pierce damage might have something to do with it as well.
::edit:: updated to reflect more real values.