The operator wasn't really using that bucket right for those conditions, although the second layer of limestone was indeed harder.
Basically the bucket is designed to rip rock as the bucket gets curled in, not dragged along, one set of teeth rip and the middle set follows and the last pair also follows. The idea is that all of the bucket curl force is applied to one set of teeth in a progressive manner, if the bucket was a traditional bucket with those teeth on it, it would take a lot longer to rip through.
But it only works right if it's started right, dig in layers or benches going down, and don't work from the bottom of the trench and dig up.