On that stage, physically and mentally, JV worked him over and got inside his head. He wasn't fooled but he did too much e.g. after getting attacked and counter-attacked repeatedly, he put in that big dig to try and drop Roglic and Vingo only to have it instantly covered. He tried to show them that he's still the boss but he should have let Roglic go a bit, recover and ride at his own tempo, only watching Vingo. Up until that stage, Pog had the measure of Vingo.
What didn't help after was the continuing super-strength of Jumbo and the lack of UAE team support.
Mentally? I think more of Pogacar than to think his mental strength is what was decisive. Why would others attacking get into his head?
Only watching Vingegaard sounds good now. At the time? Roglic after his crash was still third on the Planche. So while normally you would expect him right there with Pogacar (and Vingegaard, who was a bit of a surprise there, same time), third still ahead of the rest seemed a good sign, on the way to 100% again. And probably Jumbo didn't know if it would get better soon or worse. Certainly Pogacar and UAE had no way of knowing. Seems Roglic's back pain got worse, until he gave up. No way of knowing that on the Galibier for Pogacar and UAE. And as others have said, let Roglic win 2 minutes (to be with Soler/Majka that's what it looked it would be when they were attacking) and have Van Aert tow him to the bottom of the Granon... he starts with 3'+ advantage. Hm, that's not where you want the second of the Tour 20 and winner of the last 3 Vueltas to be really. No, he had to cover that. Then the tempo at the end, with only Vingegaard, that maybe was too much, but again, Vinge rode the same tempo. And on the Galibier riding from the front really doesn't hurt much more than from the back. Mental question if you want, some riders don't seem to like making tempo on climbs, Pogacar doesn't seem to mind in the least, he had 0 disadvantage vs Vingegaard from that. Overall he spent the same amount of energy to Vingegaard on the Galibier.
Why the big time loss? Many possible reasons, didn't eat enough? Altitude? Was ok on the Galibier, but then all his super perfomances so far were on lower altitude, maybe he does a bit less well high up? Spent too much energy? This one can explain the time loss to Quintana and company, not to Vingegaard, since they rode together until then. Conclusion: Vingegaard was just stronger that day. As he was to Hautacam. Pogacar had a small collapse on the Granon, reasons see above, a mix of all possible too.
Up until that stage he had the measure of Vingegaard? Yes... but there is big difference between the stages until then and this one.... Col de la Croix and Pas de Morgins ? Nice, but with all that's coming... Planche des Belles Filles? Castrated with the addition of Super anyway, everybody just waiting till the last meters, but again, something completely different from a Galibier northside-Granon stage.
Lack of UAE team support? Planche was bad, huge group, Majka had to stop too early. After that was ok, Soler ok on Galibier, Majka then brought back by Van Aert just in time for the Granon, Majka certainly did well there. Alpe, forgot... Pegueres Majka was there until his accident. Peyragudes nobody can criticize McNulty. Hautacam, yes, the 3 surviving helpers didn't do much. The team did ok, Majka better than I expected. Put Vingegaard on UAE and Pogacar on Jumbo... and Vingegaard still wins. (If you switch after the pavé at least, a mechanical or crash on pavé for the UAE leader....)
Plus many here pose the wrong question anyway it seems. It's not why did Pogacar lose. It's why did Vingegaard win. And the answer is, he had the best legs, he was the strongest.
Wouldn't be surprised if he does a better TT today either. Normally would consider Pogacar a bit better, but after how this went... Think they will be within 30" of each other, but if Vingegaard wins a minute I'd be less surprised than if Pogacar wins one. Motivation and all too of course.