x axis change during printing

[bmdiyh]

Hello recently I bought a Prusa i3 Pro B. Everything is working I printed out a few stuff, but during printing sometimes the x axis moves in starts the next layer, see in the picture.

14191515_961639237296216_122970460_o.jpg (78.55 KiB) Viewed 3434 times

Do you have any suggestions of where might be the problem? Thank you!