Its hardware, had this problem a while back, if you read through different websites you will see ppl have that issue. its the way the screen is built, you can recreate the issue by a applying pressure on the top and bottom edges, just to know that its not software

read a way to resolve this and it worked for me, if you have the habit of putting your phone in jeans pocket , then put it screen outwards, dont put the phone in the pocket while the phone facing toward your leg, that should stop it from doing it, i avoid putting the phone in my pocket and carry in hand most of the time,
And yea in your case the drop could have been the reason of this. but you shud try this, it worked for me, my phone has NEVER been dropped though but this issue is common with ppl having L1520