Was it on when you slipped the iPhone into your pocket? Make sure you 'lock' it before you put it in, by pressing the button on the top (left or right?).
If you are sometimes inadvertently putting into your pocket unlocked, the screen is facing you leg, and the material in the pocket is thin then you could be pressing stuff with your leg. You can tap the screen through thin materials.
Despite what everyone else said to you, I don't think that is normal and shouldn't happen inadvertently whatever you have in your pocket (since it takes wamth from your finger to actually complete that swipe). I'd either do a restore or take it back.
That may well be, but I still reckon a passcode will frustrate their best efforts, provided you don't tell them what it is. Take the phone out of your pocket before you key in the code, then they can't see it.