The decision isn't surprising. While video-first services like Netflix, Prime Video and YouTube are clearly major contributors to data traffic congestion, the sheer audience sizes of Facebook and Instagram (Facebook had 288 million daily users in Europe as of summer 2019) could cause additional strain if those users are more active than usual. Many users are turning to video chat to keep in touch with people they'd otherwise meet in person, for instance. While this is unpleasant news if you're a social media fan, it might help people work from home and otherwise get the internet services they expect.
"To help alleviate any potential network congestion, we will temporarily reduce bit rates for videos on Facebook and Instagram in Europe. We are committed to working with our partners to manage any bandwidth constraints during this period of heavy demand, while also ensuring people are able to remain connected using Facebook apps and services during the COVID-19 pandemic."