More likely to be one of the fungal or bacterial infestations Japanese maples are prone to. One party posting on UBC Botanical Garden Forums web site says sloppy nursery practice is resulting in some being sold with Pseudomonas infestations that manifest after planting out, otherwise Verticillium is commonly named as causing dieback in Japanese maples (there are other agents known to afflict these as well).

Impeded drainage is also a special hazard for these, drainage must be good. Watering every other day you may have rotted the roots. Watering frequency varies with weather and soil texture, check your specimen for its individual situation before watering. If planted with an intact field soil (often clay-like) or potting soil rootball be sure to inspect this separately from the surrounding soil as the moisture situation inside of and outside of the original rootball is liable to be different (one of the reasons bare-rooting at planting time is now being advocated by some).

Could it also have been kept out of direct sun before at the nursery, then when transplanted and thrown out in direct sun for the main part of the day it got stressed out?

The one in March probably hadn't leafed out yet and was planted at a better time,and had about a month jump start on the timing, compared to the one planted in April especially if you brought it home with fresh pushed leaves that havent hardened off yet, but i wouldn't panic just yet,being 7ft and 10 gal. that tree is pretty tough, may take a little time to get adjusted.