That is fasciation, a rather unique (and I think pretty cool) deformation of the apical meristem or primary growth point. I have never seen it in a fruit tree before (semi-common with grapes) but it can happen to any type of plant. There are apparently a number of causes, including the plant being infected with a phytoplasma, a microorganism roughly between a bacterium and a virus. But can also be caused by mechanical damage, hormonal imbalance or a genetic abnormality. In some plants this phenomenon is pretty stable and can be passed on to subsequent generations - celosia, any number of "crested" conifers or cacti and fantail willows.

Fasciation

btw, it causes no permanent harm to the plant but may impact normal growth, flowering and fruiting on the affected part. You might want to prune it off although I'd be inclined to let it be for a while just to see how it develops :-)

I have one branch of it on my apple tree junaluska. It was awful weird looking but didn't seem to effect the tree at all. I left it for one season then pruned it off.