If you create new minecart with hopper on standart rails, it will behave lin it's on powered activator rails (sucking items from chests, collecting items lying on tracks) until they touch activator rails.
Steps to reproduce:
1) Create long track of standart rails with activator rail in the middle.
2) Place minecart with hopper at one of the end of a track.
3) Drop some items on the lenght of the track.
4) Push hopper minecart to the other end of track.
All of items before activator rail will be collected, all of those after will not.