Did you register the domain name before their trademark ? There are several ways under the Uniform Domain Resolution Policy whereby they can come after the domain name. https://en.wikipedia.org/wiki/Uniform_Domain-Name_Dispute-Re...

To answer the other question posted to your question. Registering a trademark in an attempt to take a domain name from someone else is know as Reverse Domain Name Hijacking https://en.wikipedia.org/wiki/Reverse_domain_hijacking