How are the priority dates determined?
Each month, an official in the US Department of State looks at a number of statistics. Specifically, he looks at how many immigrant visas and Green Cards have been issued during the fiscal year so far to applicants from each country, and in each category. This will tell him how many are left in each category. He divides this by the number of months left in the year to find out how many should be approved in the next month.
He also looks at how many people may no longer qualify, are not interested any more, or for other reasons not pursue immigration, and how processing delays at USCIS would affect the process, and makes an educated guess of how many petitions should become now become eligible to hit the target quota.
Next, he looks at how many petitions have been filed, and when they have been filed. He then essentially counts of the oldest ones until he has enough to approximately hit the number of people who should get a GC or immigrant visa this month. This determines the priority date in the category.
All this has to be done separately for each country.
If his guess is wrong, or if USCIS suddenly develops or clears a processing backlog of Adjustment of Status applications, it sometimes happens that far more or fewer immigrant visas or Green Cards are issued during a month than predicted. In that situation, he needs to adjust the current priority date for the next month to correct for this. This is why sometimes the priority dates retrogress. If such a situation causes the whole annual quota to be exceeded, he has to mark the category as "Unavailable" (in the visa bulletin, you will see the letter U to indicate this situation).