Most probably, it's stress from many factors. Change is soil and water demand (try not to disturb the roots when up potting), humidity change (from indoors to outdoors?), light change (grow light vs sun), Temp Change, Wind stress.. etc. They can all add up and stress out the plant if too much changes at once.
Last year, all my losses were going from 10 oz cups to 2 gal pots. I don't expect to have as much trouble this year... knock on wood..