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This article suggests some possibilities: https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/brown-leaf-spots-on-buddleia.htm
You can take some samples of leaves and stems to the Extension Service, they should be able to tell you exactly. This link will help you locate one: https://www.gardeningknowhow.com/extension-search/

When you dug up the plant, you left some of the roots behind. Now there aren't enough roots to support all the leaves. The answer is to prune off some of the branches. Make sure the earth where you replant has been well-worked, so the new roots can easily grow into it, and make sure the root ball and surrounding area get plenty of water. Here's an article on transplanting that might offer some more tips: https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/tips-for-transplanting-a-butterfly-bush.htm

Is there a possibility that your plants have too much shade? That's one thing that can interfere with flower productions; another thing is too much nitrogen fertilizer - work some bone meal into the soil (to increase phosphorous) and discontinue fertilizer. Another possibility would be aphids or thrips. These articles may give you some help: https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-care.htm
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/brown-leaf-spots-on-buddleia.htm

I would suspect that a fungus is attacking the roots or leaves of the sickly plant. Treat the plant's leaves with a fungicide and mix some fungicide into water and water the plant with it. I will say that if it is a fungus on the roots, they tend to be more resistant to fungicides and may not fix the issue. In which case, it may be best to remove the plant before the fungus spreads.

You can trim away the dead parts of the plant and there is a chance your plant can come back from the roots.
Provide water and wait and see.
I have had very healthy butterfly bush plants fail to return after winter, for no identifiable reason.
Here is a link to refresh you about care of a butterfly plant.

https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-care.htm

You can grow butterfly bushes in containers, however, you will want to only do so with dwarf varieties, such as Lo & Behold Blue Chip, Nanho Blue/White/Purple, or Petite Snow/Plum/Indigo.

For more information on the care of butterfly bushes, please visit the following link:
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-care.htm