Domain-specific failures are often related to maintenance work on the domain servers authoritative for that domain, or a switch of the domain's digital signatures (as used by DNSSEC validating resolvers like unbound), or sometimes misconfigurations.
Those kind of failures usually get sorted quickly by the domain's maintainers, i.e. they don't last for long.
DNSSEC also relies on correct time information, so if your host's clock would be off by too much, consequently DNSSEC validation and thus DNS resolution would fail. However, that may not quite match your observation, as that would affect all DNS requests alike, until time is in sync again.