Hey, what's going on with real estate??????

So, I check zillow 2 weeks ago on some property and the zillow estimates are $91,000 and $87,000. Two weeks later(yesterday) the zillow estimates on these same 2 properties are $64,00 and $71,000. What is going on???????????? So that means anyone who bought property in the last 3 to 4 years is all of a sudden, overnight, underwater on the equity on their property?????? What is going on??? Does anyone have an explanation for this? P lease let me know in the comment section below. Thankyou!!!!!!!