It’s because trees want to take a shape that works for them.
Basically trees have five main things in mind.
Keep anchored (which is the job of the roots), suck up water and fertilizer (also the roots), don’t fall over (the roots plus the trunk), reach more sunlight (the branches and the leaves) and suck up sun for energy (the leaves).
There’s a lot of different ways that a tree can do all five, but it really can’t do the fifth one very well unless it spreads out sideways a little (yes, there are exceptions like palm trees). The reason for this is the leaves get in each others way, blocking the sun.
So as it grows, it splits, and this lets more leaves be in more sun because the tree starts getting more sideways as it gets taller. Now the tree COULD just keep trying to keep all its leaves and grow taller and taller without branching… but this starts making “don’t fall over” a lot tougher at some point.
So the tree branches to get more light once it gets to a certain size, and keeps branching after that to collect even more.
The top of the tree is in charge. It grows like crazy to get more sun and make more side branches. More sun and more leaves mean more sugar and trees like sugar too.
Eventually the side branches can’t hear that bossy top – they grow a little more and soon they’re telling the buds behind them to grow more side branches while they chase the sun and make more sugar.
Douglas Fir and Sequoia and Red Alder branches have really good hearing and only rarely subordinate.
Lilacs and Hornbeam and Willow never listen.
Trees that are topped go crazy – there’s nothing to listen to.
At least for some trees, there are special cells (apical meristem) at the end of a stem. These cells produce a chemical (auxin) that stops other stems from branching. As the stem grows long, there is less of that chemical so other stems start to branch off.
The first stem a tree has is the trunk. So after the tree is tall enough, a new stem branches off and the tree starts to get wider at the top. This happens again and again over time.
As it grows taller, the older stems get bigger around to help support the weight. This is able to happen because the tree keeps its food and water movement cells just under the bark. Over time, these cells are made more rigid with other chemicals and new food and water movement cells grow on top of those.
This is why we see rings when we cut into a tree also. This process usually happens on a yearly cycle, so we can often count those rings to know how old the tree is.
This is important to your original question because those special chemicals from the ends of stems are pushed around by the food and water cells. That affects how much of the chemical is in any part of the tree and isn’t a perfect process.
Since this is ELI5, I’ve definitely oversimplified. Someone linked a post in r/askscience that has the more technical names.
I don’t remember much of the technical lingo from my Aboriculture class (Ecology, Evolution, Natural Resources Degree), but the simple answer is that usually one tree stem is dominant and this becomes what you know as the trunk. What will sometimes happen is a second stem will compete for dominance and both will grow around the same rate. This causes the split that you see. Sometimes those split trunks will even fuse together and form one larger trunk, which is better for the tree.
Edit: still have my text and it confirms that whether or not a tree will actually split is due to genetics, but the reasoning above is accurate.
Edit2: I’m not suggesting 2 trunks are better than one. I was just making a point that it’s better for the tree if they fuse together, which is not likely. One trunk is always better.
Arboriculture 4th edition -Integrated Management of Landscape Trees, Shrubs, and Vines by Richard Harris, James Clark, and Nelda Matheny.
The most common cause of trees splitting is a defect inherent in a lot of tree species known as included bark.
Basically, in a healthy branch union, the bark from one branch meets the bark from another branch and grows outward, forming the branch bark ridge. This normally happens in wide or U-shaped unions.
When the union is too tight, or V-shaped, the bark from both branches meet and grows inward. Eventually, this will cause weakness or a split in the timber below.
You can mitigate this by bracing, but it’s rarely successful. The maintenance of bracing (as with most long-term plans for managing trees) needs to be multi-generational for the owners. More often than not, the brace is installed then forgotten about.
Decay fungi can also cause failure, but this is more snapping than splitting.