Any idea why the chain fork occurs?

Any idea why the chain fork occurs? And how to avoid it?
Around 12 April 2024 a chain fork happened.
There were unavailable boot_nodes, so I used the IP of the blocks explorer as a node and was successful - I got the same height as the blocks explorer.
But yesterday, for some reason one of my nodes switched to a version of the blockchain that has less height and doesn’t match the blockchain explorer. And it’s the exact same chain from a week ago.
Someone is mining that chain as well. And now I’m questioning which one is correct.
Any ideas?

Keep your node updated: