I am thinking the browning could be due to a couple of reasons. One might be the natural life cycle of the tree, where the intergrowth sheds as new outer growth develops. I dropped an illustration from Michigan State University that highlights this principle (image attached).
Another reason could be due to intense sun exposure. Depending upon where the tree was initially grown, some junipers can turn brown when exposed to long periods of direct sunlight.
Another possibility might be overwatering. Junipers do tend to like a bit drier soil, compared to other trees.
If nothing has changed and the tree has been growing well for some time, I would think that the first reason might be causing the browning. Essentially, this happens as the inner needles and branches aren't able to get as much sunlight as they used to.
If it is the second, the solution for this would be to place the tree in a partial shade position. Peter Chan, owner of Heron’s talks about this in a few of his books.
If you believe it is related to the third reason, you might get a moisture meter and water the tree only when it's just become dry.
Another reason, which I didn't mention out of the gate would be whether or not the tree has some sort of infestation like spider mites. You would have to do a close examination and see if you can identify any little critters on your plant.
One more thing to consider is stress, which might cause browning if the tree was moved into a new environment recently.
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