It's the combination of the design and the weight of the arms bearing a constant "pulling" pressure on two small screw holes...even my beloved Citrus are starting to show an ever so tiny hairline crack on the left side.
I failed to mention that i have a second pair of the blue 519s and they too have a feather crack beginning to show around two of the screw holes. not as big as the cracks that developed on the other 5 Elevation era Bulgaris I owned (2 blue - 519 and 620?, one red and 2 flash 519s), but then again I haven't been throwing this 6th pair aside after they fog up, I've been gingerly setting them down. That and I no longer let people try them on, that creates strain as well especially if their head is bigger than mine (which, seems to be an impossibility but....lol). Someone posted a picture of some "bono-type" Bulgaris a few pages back, pre-Elevation era 635s I believe they were (EDIT: it was BADGER who posted them back on about page 15), the first blue pair he posted, they have kindof a square big lens..if I'm not mistaken those were 635s circa 2000/2001)...I also owned a pair of those, they were actually the first Bulgaris I had given to me by my good friend Mark. THey too broke. Bulgari didnt have very good screw assemblies back then I guess.
Whatever you do, RESIST!!! with all your might the temptation to tighten those screws if they seem to be working their way loose, that will definitely contribute to the deterioration. Only let an eyewear pro you trust do it for you, and even then if they aren't aware of the inherent flaw in the design, make them aware and ask that they be ultra careful in tightening.
All told I think I've had about 8 or so pairs of the various Bulgaris over the past 5 years and all of them have developed some type of problem. That's not a great track record when weighed against the super durability of the Black Flys, the Romeo Giglis which are also virtually indestructable, the D&Gs which I have also had zero problems with, and, perhaps surprisingly, the Armanis, which have also held up very well under some fairly strenous use. I never owned the Mius because I never liked them but I can't imagine that they'd be all that durable, but I could be wrong. The Versace 2040s don't even get on the radar for me, they're ugly as hell and he'll never wear them onstage anyways so I haven't paid any attention to them.