Hakaijuu

Hakaijuu (ハカイジュウ, Hakaijū) is a Japanese manga series written and illustrated by Shingo Honda.

Plot
You lead quite a nice life... Competing with your friends... Having a childhood-almost-girlfriend, who is coming back to town... All dreams starting to come true... when the ground suddenly shakes and a shelf drops on your head....

Well... You hit your head just now, so it'd be quite... reasonable that everything went nuts and people get eaten before your eyes, wouldn't it?