Those cracks are known as hard point fractures.
Fiberglass is very flexible except at sharp bends.
Any time a flexible fiberglass panel meets a hard point (sharp bend),
the possibility of a fracture occurring goes up greatly.
The transom is laid up as a single fiberglass skin
with the rest of the hull. The floatation compartments
and benches are added afterwards and are not built as well
as the rest of the hull. So any hull flex transfers unevenly to
the components glassed in later. Your fractures appear to
be caused by the weight of the outboard shaking the transom
which has caused the separation fractures you are seeing.
The transom is peeling away from the benches.
I can tell this as there are no cracks on the outside of the transom,
and the symmetric fractures to the tops of both benches.
Is the bottom of the transom and hull properly
supported by the bunks on your trailer? If not, after making
the repair, those fractures will occur again.