Minecarts are pushed by the block moved by a sticky piston which are one block above. After a short delay, the minecart get back into it's initial position
Environment
java version "1.8.0-ea"
Java(TM) SE Runtime Environment (build 1.8.0-ea-b84)
Java HotSpot(TM) 64-Bit Server VM (build 25.0-b25, mixed mode)
Windows 8 64bit
Caused by wrong collision box position.