Why is my lock screen activating randomly?

[Irisse Ortiz]

I have a S cover for my S4 and the lock screen for the s cover activates randomly

Whenever i open my snapchat and open my snaps, a black screen [S cover lock screen] pops out with the time and date, its as if i put the cover on (which i didnt) .. this also happens randomly sometimes. What do i do? :/