Narrow angles in a tree are weaker than wide angles, and joints with no ridge have bark included between the branches. The included bark not only keeps the two sides from growing together, as the tree grows the branches push against one another and eventually will push the tree apart.
This problem is quite common in shade trees, partly because of tree species choices and partly because shade trees usually don't have enough competition for sunlight, and they spread rather than growing up straight.
They may break up in storms, or on a dead calm day, like this Bradford pear tree.
The wounds caused by these breakups are huge, and the tree really needs to be taken down before rot makes it even more dangerous.
Ignore the warning signs and you probably will be going out in a storm to open up your driveway. When pruning for timber production, the first pruning done is to make young trees merchantable when they mature. It may just be a nip or two to make a young junk tree into a valuable tree. Shade trees are similar. That early pruning can make a tree a good, safe neighbor for you. Failing to do those cuts can cause damage and inconvenience in the future for you.
