prune a rosemary bush
It looks like the weight of the branches are doing this, but rosemary can get extremely woody and easily overtake and area without regular pruning. I would wait until mid-March to do any serious pruning, but if you want it to have a mounded shape, then you need to regularly prune to maintain that shape. What I would do is take a long look at the major branches and cut off those where the plant is spreading in a direction you don't want it to grow. If it is looking thin in the center, then prune to where you can see new growth directed where you want to fill in.
Most woody herbs won't send out new grow on bare wood stems (this is especially true of lavender, but rosemary is similar) and you will see that leaves drop as they get shaded. If you regularly prune to make sure the plant has an open structure, you make sure that the leaves that are there get plenty of light. With rosemary, you can easily snip out every other branch and still have a full-looking plant.
The good news is that if you go a little too far with the pruning, you can always purchase a new plant for not a lot of money and start over.
Thanks so much!