Three Atlas 5 Centaur upper-stage breakups in GTO in less than a year, and the cause is a mystery

UPDATE Aug. 3: NASA's Orbital Quarterly Debris News makes reference to the latest Centaur upper-stage breakup, in April, but says no debris has yet been entered into the catalogue for the event. Here is NASA's statement:

"A Centaur V Single-Engine Centaur (SEC) rocket variant (International Designator 2018-079B, SSN number 43652) fragmented in early April 2019. At the time of the event the stage was in an approximately 35,092 x 8526 km altitude, 12.2° orbit. This Centaur V upper stage is associated with the launch of the USA 288, or Advanced . . .