Minecart's movement is a little bit buggy in 1.8. Try it out and you'll see.
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Is this still an issue in the current Minecraft Snapshot 15w45b or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
Ticket resolved as incomplete, because no answer in a reasonable amount of time (1+ year), if it still happens, please update the ticket.
I have had this problem recently as well. There have been many glitches with minecarts, and the most annoying one is the rotation of minecarts being reset after summoning one with a preset rotation.