it syphons back because the opening at the sump pump end is lower then
the opening at the other end. work the same as a syphon

I suggest you install an anti syphon check valve at the pump which will
prevent the water from running backwards.

you can also add a SMALL hole to the pipe at the highest point after
the check valve to allow air to enter and let the water slowly drain
out of the pipe when the pump shuts off so it is less likely to freeze.

Yes some water will come out of the small hole when the pump is
running.

Mark