Flare up is exacerbation after endodontic procedure wether retreatment or treatment
both I think are the same

the causes of flare up either microbilogically , physically or chemically
you may read more about flare-up in a review by Richard Walton 2002 Endodontic topics
Torabiejad 1988 JOE
Ric. Walton & Fouad 1992 JOE
Morse et al 1986
Samuel Seltzer 2004 JOE
S. Oilet 2004 JOE
Samuel Seltzer 2004 JOE wich is a re-review to his original article in the 80s
P. Eleazer 1998 & 2004 JOE
those are the key people in flare-up
good luck