I think what's going on is that after a year at any job you start to see the problems with the code base, and it starts to get repetitive. It will be like that both in big-co and small-co, but in a different way. In small-co you can have a much more freedom, but you reach a plateau of the new things you can learn. In big-co there are bigger problems and more challenges, but you're part of a soulless machine that just wants you to do your job and not ask questions.
Not having experience the soul-crushing big-co life, the brough-up-to-speed junior hires don't appreciate the deal they have, and want to explore their options. It's a rational decision, and come to think of it I'd probably do the same. The code is always greener on the other side... Who wants to be like "no I'll stay here and do 3 years of the same things that I'm really good at now and delivering value for my employer."
From the employers's perspective though was it worth it? Time/energy invested to train them vs. maybe 6mo or 1 year of useful coding contributions.