Wall of text incoming explaining this totally "normal" process in the egg-industry:
Tap for spoiler
(Translated from german, source linked at the end)
Egg laying hens in conventional agriculture have a short and deprived existence. After only ten to twelve months of intensive egg production, in which they are exploited to the limits of their performance, the so-called "Mauser" follows. However, this natural resting phase, in which her body regenerates and renews the feather dress, means for the hens the end of their "usability".
The Mauser goes hand in hand with a laying break of at least three weeks, which means an unacceptable slump for the profiteers of the animals. Instead of giving the hens this well-deserved recovery, they are mercilessly eradicated. In Switzerland, 3 million laying hens suffer this sad fate every year and are gassed or otherwise killed after only a few months of their lives to make way for young, "more capable" chickens.
(source: swiss-german animal-protection website)
Note: This is one of the reasons people (often vegans) avoid consuming eggs or products that have eggs in them.