what would cause a piston and connecting rod to sheer in a stihl 4 cycle string trimmer ?

Via the connecting rod breakage? Is there enough oil? Has it been changed every 25 hours?

In general, con rods tend to break from stretching and that can happen from running at very high RPMs over long periods of time. Another reason might be running it on a very rich mix that will make the cylinder  warm up slower and the piston will tend to "stick" and the con rod will have to work harder to pull on the piston. The effect of that excessive stretching accumulates and fatigues the metal, and it breaks at some point.