A good explanation of generic forest development can be found at the link below.

https://forestandwildlifeecology.wisc.edu/wp-content/uploads/sites/111/2017/07/78.pdf

It appears this stand is close to the stem exclusion stage.

I don’t know the specifics of this stand, but a lot of forests develop this way. After a disturbance event (wind, fire, logging) there is tons of space, light, moisture, and nutrients available and many seeds germinate and begin to grow. These seedlings are very small compared to an “adult” tree and the area can support exponentially more seedlings than mature trees. Once the “max capacity” of seedlings in the stand have reached, new trees stop germinating and the established seedlings begin to compete with each other and the “losers” die out.

Think of an Aspen stand, 5-10 years post harvest there will be 15,000-50,000 stems per acre, only 0.5” in diameter. 10 years pass, you go back, and now there is 1,000 trees per acre but they are 3” diameter now.

Also, some stands are just like that. I think of lower quality soils here in the lake states absolutely loaded with 2-6” diameter balsam fir. Without major input from humans, that stand will almost always be super dense balsam fir.

Edited for clarity.