It could be so many different things:
greater transplant shock
different micro climate
Same cultivar but different source nursery so really a different plant since different root stock and grafting scion
You watered the new plant differently and one may have dried out
One may have been out of the nursery pot too long and triggered off the leaf drop
Soil acidity
etc., I'm, out of ideas

All of the above are very viable possibilities. Regardless of the cause, leaf drop at this time of year is not unexpected and is generally of little significance, other than as an indicator of stress. As long as you to continue to provide sufficient water throughout dry periods, your tree should leaf out fine in spring.

I planted a Royal Red Maple in June of this year. It also lost it's leaves in early October. Mine was watered regularly, so I know it wasn't dry (I didn't overwater). My nursery only guaranteed the Royal Red until October 31st of this year (all other trees I purchased were guarateed for a year), so I have them coming over to check the tree next week. I did scrape the bark off a small branch and saw bright green, which tells me the tree is still quite alive.

I hope your tree is okay! I'm hopeful that this is normal, as gardengal48 says.