I really don't know the answer to the question of who/what caused the current collapse in egg prices. In theory a market eflects demand and supply, so when supply is higher than demand, the price paid by the retailer will be lower than if supply is lower than demand.
This model is easy to understand but reality is never quite so simple. Seasonality, marketing and imports/exports can also affect price/cost.
What suppliers must never forget, (but they often do) is that the core purpose of the retailer is to maximise variety offered to the customer whilst minimising the cost of purchased product. On the other hand the core purpose of the producer is to minimise variation (and variety) whilst also minimising input costs.
In the case of the egg producer his opportunities to minimise input costs are a lot less than those of the supermarket, at the same time the egg producer has less opportunities to expand his market (he has fewer customers and products) than has the supermarket. Nor is it as easy for the producer to find alternative supply sources as it is for the supermarket. (The supermarket can buy eggs/anything in from the global market) whereas the single producer has limited alternative sources of supply.
In other words. The whole system is stacked in favour of the supermarket.
In my view. Every producer of every type of food stuff should have the long term aim of doing their level best to encourage systems of retailing that can compete the the supermarkets in the locality of that producer.
I am not anti supermarket. But I am in favour of localism (not parochialism) in the food market.