I am a geologist, NOT a dirt contractor. However, the only times I have seen or heard of a similar situation is where you had a water saturated area beneath the equipment, or a backfill of some material (old logs, a trash landfill, bog, etc.) that was later covered over with topsoil.

I assume there was nothing like the second option in that location before you started construction of your pond?

If not, then I think you have very saturated clay that is probably overlying a water-bearing layer of sand or gravel. The clay is so wet, that is now has high "plasticity" and can actually move since you have now stripped off the confining overburden.

It looks like there is a large excavator on location? If so, I would definitely dig a test trench. It doesn't even have to be within the pond boundaries (if the operator is worried about the bouncy clay). Go to just outside the pond edge adjacent to a good bouncy area and dig a quick test. Look at the material that is coming out in the bucket, but also look at the side walls of the trench. You should see some layering. (Lack of layering MIGHT mean backfill.) Do NOT let a person go into the trench since wet clay/sand underneath can lead to a trench wall collapse.

How deep is your projected pond depth? I would go at least 4' deeper than that in the test trench.

Let us know if you get down to "soupy clay". If you do, then keep digging deeper. I suspect you will find some type of water-bearing layer. That will probably change the construction plan for your pond. I don't know how the contractor bid your pond job. Do not let him refuse to find out what is underneath. IMO, a poor contractor wants to do the minimum job and get out of there when presented with a strange situation. A good contractor wants to know exactly what is causing the strange situation - so that it can be dealt with in the proper fashion to deliver a good final product.

Everything written above is speculation by a person that is NOT on location. Further, it was written by a person that does NOT have any local knowledge of the subsoil conditions in the region. A local contractor does have experience with local conditions.

Good luck with your pond project! I hope I have given you a little bit of helpful background as you continue work on your pond.