I wonder if it is to do with the structure, that this is normal behaviour on here?
With centralised competitive platforms they grow by essentially destroying each other.
With federated open platforms on the Fediverse, they grow by helping each other.
When Mastodon recently gained millions of new users, that also greatly helped other Fediverse platforms, because each new Mastodon user was a potential person to interact with on the other platforms too.
For example PeerTube accounts started getting more followers and viewers because PeerTube accounts can be followed and interacted with from Mastodon.