Answer: Pine needles do turn brown and drop naturally. However, transplanting, fertilizing and soil moisture could also be factors. The older a plant, the more difficult the adjustment after transplant. In addition, if not applied correctly, fertilizer can "burn" causing needles to brown and drop. Austrian pines as a general rule prefer dry conditions, with well-drained soil, so if roots sat in wet soil for an extended period, that could be a cause.
Finally, Austrian pine is somewhat susceptible to the Nantucket pine tip moth. In summer, the branch tips turn yellow then dry and brown. The tips are "mined" by a small pinkish brown caterpillar that is the larvae of the moth. Diplodia tip blight looks similar, but won't have the hollowed out tips. I mention these two because it wasn't clear to me from your description where the branches were turning brown.
Since you probably have quite an investment in this mature tree, I'd check with an arborist or the company that transplanted it for you for a diagnosis. Good luck!
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