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It could have been deer if you have any in the area.

Well, this would be difficult to do. Either remove the primroses from the lawn and transplant elsewhere so you can keep the yard mowed or allow the primroses to stay and forego on the mowing. Of course, if you leave the primroses, you should know that they will just continue to reseed and eventually cover the area. Mowing them down will help keep them under control before they set seed.

You are in Hardiness Zones 5a-6a I see. Usually it is best to do the dividing and transplanting in the fall when they have gone dormant. However it can be done in late May as well, usually about a week after Mother's Day. Once you have transplanted them, water them in with some water with a product called Super Thrive in it as well as some root stimulator. This will help with transplant shock and get them going well.

Just cut the spent blooms or seed pods and allow them to dry out some. Then you can place them in a paper bag and leav it in a cool, dry location for planting next season or plant them now.

These plants do get somewhat tall but unless they are falling over, it's not necessary to support them. If you would like to stake them, however, it will not hurt to do so. As far as this helping them to grow taller, plant supports have nothing to do with that. An abundance of nitrogen fertilizer will cause lush growth, though too much will inhibit blooming. I would stick with either a balanced fertilizer or one higher in phosphorus.

Usually primrose are planted in the fall or spring. This articvle might help:
https://www.gardeningknowhow.com/ornamental/flowers/primrose/growing-primrose-primrose-plants-in-your-garden.htm