A "preventable tragedy" means a sad or disastrous event that could have been stopped from happening if certain actions were taken or precautions were followed. Full definition
Every time I hear of a toddler or young child drowning, I think of how preventable that tragedy is. (parentingsquad.com)
(Yes, such preventable tragedies do occasionally occur.) (themeowingvet.com)
And with preventable tragedy as a catalyst that no one wanted, a brand new hockey team has become intertwined with a sense of civic pride in the desert. (sbnation.com)