Gelcoat cracks occur because fiberglass and gelcoat have different characteristics when it comes to hardness and flexibility. Fiberglass resin is always softer and more flexible than gelcoat.
In many ways you can think of gelcoat like you can the glass fibers in fiberglass. Thick glass, like you'd see in a window pane, can flex some, but not a lot. An example of this is when you're sitting in your home at night during a wind storm and see reflections from things inside your home bend and change as the wind forces your window to flex a bit. Make the layer of glass very thin, like a strand of fiberglass thin, and the glass can bend a lot more. A layer of fiberglass cloth is as flexible as the denim in a pair of blue jeans because the stransd of glass are so thin.
The ideal fiberglass-gelcoat combination is to make the fiberglass thick enough that it is less prone to flexing and the gelcoat thin enough (one or two layers of paper thick) that can bend with the small amount of flex the thick fiberglass still has.
Cracks arise when the underlying fiberglass is thin, making it more prone to flexing, and/or the gelcoat is thick, two or more layers of paper thick, and the gelcoat can't bend as much as the underlying fiberglass does.
In trailer construction, the gelcoat gets sprayed into the mold first, and its thickness can vary quite a but. You want the gelcoat layer to be complete and uninterrupted, and it's easy to spend just a moment longer spraying it into an area than you need to. Then the fiberglass gets blown in on top of the gelcoat, and it's equally easy to get complete fiberglass coverage on top of the gelcoat, but still have some areas be just barely thick enough to cover while other areas are good, thick, and solid. When thick gelcoat meets thin fiberglass and you apply force, the thin fiberglass will flex while the thick gelcoat will resist that flex. The result is a broken-window effect with shards of gelcoat still stuck to their underlying fiberglass.