Then you should be asking yourself what changed in your/its location recently. Since pines are infamously slow responding, the event could have been several weeks ago.
Have your trees been in exactly the same places through the winter and up to now? In other words, is it correct to assume that your red pines have had the exact same treatments as your black pines?
In my experience, too much water when new shoots/needles are extending will cause the needles to turn yellow (permanently). My climate is quite cool, rarely getting much above 20C. Possibly this yellowing doesn't happen in warmer climes. On the other hand, I can't say whether this droopiness ever occurred on any of my trees - I can only say that it never caught my attention. Maybe this is a phenomenon that only occurs in warmer climes.
Mostly this yellowing occurs with my lodgepole pines but it does also happen with my young red/black Japanese pines that are in shallow pots (not in deeper pots). I use Turface MVP for everything. This year I put roughly half of them in much larger particle sized pumice and those in pumice didn't exhibit the yellowing. Are your red pines in the same substrate as your black pines and are the pots/substrate equally deep on all? Do you water all of them simultaneously?
I've also noted that overpotting makes matters worse in this regard. Did you repot the red pines this spring? Did you "up-pot" them?
Loss of turgidity (which what might be behind your tree's new growth hanging down) is usually a characteristic of something going on with the roots that limits water uptake = roots drowning, roots drying excessively, roots dying (fungal pathogens or bugs such as root aphids). Have you popped the root plug out of the pot to take a look?
Elongation is driven primarily by auxin, as is gravitropism. Your tree has elongated new growth but doesn't seem know which way is up or at least to act on the information as it would normally, by pointing the bud tip up. Gibberellin contributes to elongation; in fact it was discovered from a mold/fungus that causes rice plants to bolt. So maybe there is a similar mold/fungus stimulating overproduction of gibberellins in your tree causing it to produce long new shoots even though you've so strongly withheld nitrogen that it has an auxin deficiency (auxin is a nitrogenous compound). Has there been any 'recent' spraying of fruiting bushes/trees, say, near you? Is there myco in the substrate of your drooping red pines that doesn't seem to be in that of your blacks?
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