I am no expert, just have to much time on my hands to think about it whilst working, anyone want to let me know if im on the right track with my thinking.

If the clearances are spot on a higher spring pressure shouldn't be an issue as it should follow the cam lobe smoothly. If the clearances are wrong or the cam lobe is faulty then a change in clearance could cause the spring to snap back into the seat instead of smoothly following the lobe. If you use too low a spring rate then the valve could float on the lobe at some point creating to much clearance for a moment which results in the valve snapping back into the seat. Snapping back into the seat too many times could cause the valve to fail like it has.