These Wyverns will use an ability called Dispelling Wind every once in awhile during Besieged...and it can and will dispel your Reraise effect (or anything else you happen to be buffed with).
Judging by the picture, I am assuming this wyvern is by itself, not a summon from some sort of Dragoon mob? Just questioning because the ToAU areas seem a bit weird (havent visited yet) i.e. the Morbols...