Why does my iPhone shut down?

When I touch my screen a little hard and let my finger stay there for some seconds, some white and black lines appear and then the whole iPhone shuts down. After some seconds the apple logo appears and the mobile turns on again.

Can someone please tell me why this happens, and what I can do about it?

Could be a virus. Scan it with antivirus software.

http://comparefroggy.tumblr.com/post/56876301264/whats-the-best-phone-in-the-market

May be a hardware problem consult an imagine service centre
if it is a software fault just backup it i'm not sure just try. If there's any carsh in software also it may occur.

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