You have likely seen a topped tree, which are trees pruned to have an unnaturally flat top. Topping is both ugly and it's detrimental to the health of a tree.
Topping Reasons
The main reason trees are topped is due to the size of the tree. In many cases, the issue is with a tree that hasn't been properly trimmed in some years. Due to its large size, the top is cut off in an attempt to lower the height and spread of the tree. Although the tree height will be lower, the topped tree will have a blunt, flat top and a lot of branch stubs.
Sometimes homeowners have young trees topped as well. This is usually in a misguided attempt to slow the upward growth of the tree. The trees are usually cut with a flat top, and often all the secondary branches are also removed.
Tree Health Concerns
Topping is more than an aesthetic issue, as it compromises the health of the tree. The blunt branches typically aren't cut back to a bud properly, which results in dead twiggy branch ends that are more disease-prone. Interior branches may not be thinned, so the tree has poor air circulation and a flat top that allows moisture to collect. The result can be an increase in fungal issues.
There are also structural issues. Topped trees will quickly put on weak growth to counteract the branch loss, which can lead to branches that break more easily. The weak structure of a topped tree can also be more prone to splitting. Loss of leaves due to extreme topping may lead to nutrient deficiencies, which can further weaken the tree.
Alternatives
You can avoid topping with proper pruning. The best way to control tree height is to plant a tree with a mature size that fits into your yard and to make sure it is properly pruned each spring. Failing this, you can have the height of the tree gradually reduced over the course of three years. Your tree service will cut back each branch by 1/4 to 1/3 its length, taking care to maintain a properly rounded crown.
Young trees should be pruned selectively instead of topped. Weak branches are trimmed out so only a strong network of lateral branches growing off a central trunk is present. All cuts are made back to a growing bud so there are no stumpy branch tips poking out.
Contact a tree trimming service if you need more help with your landscape trees.