Whenever any minecart is on a detector rail (powering the rail) and some piston contraption causes both the minecart and the detector rail to be pushed (works fine on pull, crashes only on push) then MCPE is likely to crash. The crash does not happen every time, and with different builds, it happens more or less frequently (suggesting this bug has something to do with redstone tick order of operations).

To reproduce

1. float a piston (sticky or not, doesn't matter) above the ground pushing a slime block horizontally
2. attach any solid block above the slime block (so the piston will push this block because it is stuck to the slime block)
3. attach any solid block on which a rail can be placed beside the slime block, in line with the piston (so the piston will push this block too)
4. place a detector rail on the previously placed block (the orientation of the rail relative to the piston push direction seems to matter. If the rail points perpendicular to the direction the piston pushes, the game crashes more frequently.)
5. place any minecart on the detector rail, activating it
6. activate the piston by any means
7. MCPE has a high probability that it will crash to menu

Note: The above build is not the most general case, but does crash with high frequency. Slime blocks are not required to reproduce this crash. You can also stack two pistons, use redstone to trigger them at the same time, place one block in front of the bottom piston, place the detector rail and minecart on that block, and activate the group of pistons. This is illustrated in the second screenshot showing the pistons and the rail pointing the same direction as the push.