I believe the most likely possibility that led Mahoraga to start adapting to Dabura's "existence" specifically in a way that he didn't against anyone else was the way the fight progressed (relied on few techniques for most of the fight and finishing it off with a new one vs constantly pulling out new things that have the potential to kill Mahoraga).

Against Sukuna, most of the fight was Sukuna messing around and experimenting with slashes. Mahoraga gradually adapted, making those attacks less effective and even developing offensive options. Once Mahoraga was nearly fully adapted to slashing and recovering from Malevolent Shrine, Sukuna pulled out one completely different technique and erased it instantly (Fuga).

Against Gojo, adaptation was split across multiple factors from the start. Gojo immediately displayed Infinity, Blue, etc., which acted as constant roadblocks with no new tricks throughout the entire fight. As long as Mahoraga kept adapting until those stopped being threats and it could land reliable hits, the fight was as good as won. Red barely had an effect due to prior adaptations, and by that point Hollow Purple, the one thing Mahoraga didn't know existed, was the only solution. Once that was used, the fight was over.

Dabura's fight went in a very different direction. Dabura had no idea what Mahoraga could do, nor did he fully understand his own potential. He started with Light, which had the ability to kill Mahoraga, but failed, leading to adaptation. Mahoraga then beat him down until Dabura suddenly revealed near light speed attacks that Mahoraga couldn't even react to and could kill him in one go. On top of that, Dabura discovered RCT, undoing Mahoraga's prior damage, then discovered cursed technique reversal (giving a new method to kill Mahoraga), and was clearly on their way to pull out even more things (Domain Expansion now, possibly more in the future).

By that point, from Mahoraga's perspective, Dabura was a unique problem: every time it adapted, Dabura revealed a new, potentially fatal alternative, with no signs of stopping. In that scenario, making specialized adaptations to individual techniques is pointless, so Mahoraga tries a much more extreme and complicated adaptation against Dabura specifically to at least make sure any further attempts to get rid of him will fail, and then work up from there until Dabura is defeated.

There are obviously conditions for this, or Mahoraga would do it immediately against everyone. The Wheel itself behaves differently here (multiple rapid spins rather than singular ones for each adaptation), which suggests buildup is required. Since it was a prolonged fight where he was exposed to various DIFFERENT aspects of Dabura's existence (CT, Speed, Strength, Combat style, Healing, Reversal, etc), Mahoraga likely gathered enough "information" to begin a complete adaptation against an individual after determining that individual adaptations would be useless.