Answer:
It depends, there needs to be more context in order to pinpoint the exact answer. A short list of possibilities are:
1. There was a shortage of milk this year, so they need to increase the production to balance out the sales.
2. They found that selling milk rakes in a good amount of profit, so to increase profit rates they increased the price.
3. They found that more people started buy milk, meaning that production costs increase, so the price does the same.
If you would like more possibilities feel free to inquire :)