Zinnia flower root formation!

I planted some Zinnia seedlings into pots last Spring and thought that I would pull them up so that I could plant some replacement seedlings.  They wouldn't pull so I tipped the pots upside down and was surprised to find that the root balls had practically filled the pots up.  I would welcome some comments on that because I've never seen anything like it before.  If I had left the cut back Zinnia's alone would they have re-flowered?