My first guess as to the cause is that the difference in expansion and contraction of the metal windshield frame and the fiberglass causes excessive stress on the mounting screws. It is also possible that there is excessive stress due to windshield frame movement from possible using it as a handrail when going and working forward or even just strong forces from wind. The mounting screws holes are common stress points since the screws themselves create a 'splitting' force when installed. It is possible that the holes in the frame are too tight (narrow) and don't allow any free movement of the frame around the mounting screws.
The simplest approach would be to grind out the cracks, sand the surrounding areas around the cracks and apply a epoxy filler, either pre made of made with epoxy resin and something like Cabosil. After curing and sanding, gel coat would be applied, possibly after a fairing compound and then sanded, polished and waxed. A decent fiberglass shop can make the cracks disappear. Don't try to fill the cracks without the grinding process, the filler will fail. The cracks need to be opened a bit to expose fresh fiberglass on both sides and the bottom of each crack.
One thing that might concern me is that the cracks may be new and developed quickly. Normally I would expect to see some rust bleeding near where the cracks meet the frame. Moisture will seep in and usually causes the screws to rust (crevasse corrosion). If the cracks developed quickly there might not have been enough time for the rust to weep yet.
Regarding the removal of the windshield; That might be the needed approach especially if there is something structural going on other than normal expansion and contraction but that removal can be complicated and there is a good chance you will run into snags. I don't suggest that approach. Patching the cracks may last a long time but if it doesn't then you may need to think about frame removal.