Oak Tree Question

Hi,

I am an inexperienced gardener so apologies in advance for the basic descriptions and possibly basic question.

I moved to a new house just over a year ago. I have two lovely old oak trees in my garden. 

When I moved in the trunk of the trees and the main branches were what I would call clean with all the leaves being at the end of the branches.

Over the last year there has been lots of growth on the main trunk and branches. The trees have filled out a lot with new growth. I preferred the cleaner look of last year.

I'm assuming the trees are about 100 years old and it has me wondering why the growth thickening out the trees has happen in the past twelve months.

I'm wondering if this is a cyclical event or have the previous owners been pruning the new growth regularly?

I hope that makes sense.

Thanks!