The "stepping back", as mentioned in the title, doesn't summarize what's happening here. laanwj decided to delegate some of his lead maintainer tasks to others with aim to further improve the decentralization and reducing the attack surface. He want to continue to work on Bitcoin an Bitcoin Core.
Most recently, this became relevant as the bitcoincore.org website he (co-)maintains received a legal letter from a (fraudulent; as in unproven and repeatedly lying) entity claiming copyright of the 2008 Bitcoin whitepaper. He complied[0] by taking the PDF down from bitcoincore.org and received a lot of slack for it by parts of the community. He then made clear that he'll happily continue to work on Bitcoin Core and Bitcoin in general, but will not be a "martyr"[1] for the project: "it's up to you as bitcoiners to protect it". As a reaction a lot of community members and companies proceeded to host the PDF on their site [2].
[0]: https://github.com/bitcoin-core/bitcoincore.org/pull/740