Their domain expired because at some level people made some pretty boneheaded mistakes.
Whomever their actual registrar actually was (GoDaddy it seems) stopped pointing the zoom.us nameserver record (NS) at AWS Route 53 which Zoom obviously uses.
% dig +short zoom.us NS
ns-387.awsdns-48.com.
ns-1137.awsdns-14.org.
ns-1772.awsdns-29.co.uk.
ns-888.awsdns-47.net.