Turning off the App Store gesture?
I think you're opening Slide Over multitasking on an iPad and App Store happens to be the last app you used with Slide Over.

Instead of sliding from the very edge of the screen, just slide anywhere else on the screen. Start your finger somewhere on the display and Slide Over won't be opened.

You can disable Slide Over completely: Settings → General → Multitasking and turn off Allow Multiple Apps.