Mutant

This is a mutation. Plants arent meant to grow this flat. This is a type of physiological mutation called Fasciation. It can happen to almost every type of plant out there. It is where the stem grows abnormally flat with the occasional leaf growing out of the stem and a cluster of smaller shoots on top. I wouldnt panic if you see this. Its just the plant being a freak, if you research it there is no cause to it, the plant just does it. In my 9 years of gardening experience, Ive found that spraying Roundup near a tree/large shrub can cause this, but its hit and miss. To fix it, you just prune it back to normal stems. Ba Na! Problem solved.