Because our spring had whiplash weather, alternating between extreme heat and cool, wet weather, it could be that your 8-week old Junipers are still in the midst of transplant shock, dropping old needles to ensure that new needles can grow.  As you said, it's hot!  Even with shade, the new trees are struggling with the heat.  It's usually better to plant junipers and other conifers in the fall so that they can settle in during cooler weather and don't get cues from daylength and rising temperatures to put out new needles and instead put energy into roots.

Drying and dying needles on your established bushes may be winter desiccation.  When a conifer doesn't get enough moisture during the winter, the needle die back from the top down and outside in.

These are both water issues, but compounded by the dramatic fluctuations in weather.  Your watering schedule sounds fine in a steady state, but you may want to adjust to the intense heat with the new Junipers, watering more deeply during extreme heat.  Also remember to winter water once a month in the winter to help prevent the bushes from drying out.

https://extension.colostate.edu/topic-areas/yard-garden/fall-and-winter-watering-7-211/

Q-#0144444  Dead Needles- Juniper Shrub

I'm following up with a huge "thank you" for your reply.  I'll use the advice in the context of planting future shrubs; plant in the autumn for best results in thriving.
Shayne Brady , Denver