It's called zippering. Usually a result of pollination issues during cooler weather; similar to catfacing.

Dave

That's strange, since we had temps in the mid 90's to lower 100's this summer. Maybe the same pollination problems are present in hot weather too?

LOL! Love it.

Many of my San Marzanos look like that too!! This is my first year growing them, so I thought that was normal.

Hi hcoon,
It was my first year too. They almost look like they grew through some chicken wire fencing. They are weird, but cute! haha