It's his final phase when his health bar is less than a quarter. I noticed that too after practicing fighting Vergil in bloody palace warm up so many times. And it's the only time when he'll do his JCE at the end of his combo, in his SDT form.
As someone who plays Mission 19 all the time, this triggers during his third phase, or when his HP bar is on its last quarter. The indicator of this happening is the cancelled dive bomb move. Expect heavy rain swords and storm swords, then an upper slash, three aerial cleaves, the second one triggering blistering swords. Next are judgment cuts, two rapid slashes, the second one going straight into a JCE. That's the pattern for it mostly. Amazing Nero gameplay by the way!
This is normal, he actually isnt fighting has hard as he usually fight me, the first time i fought him it took an hour straight, i won but jesus, all i could do was clash with him for 20 minutes at a time with nobody getting a single hit in, idk why he was such a tryhard the first time, but even now he still tries harder than in this video
