Zenimax Media( Owner of both Bethesda Studios) trademarked the name “Hearthfire” under goods and/or services for”Computer game software for use with computers and video game consoles; downloadable computer game software offered via the internet and wireless devices”

Source: http://tarr.uspto.gov/servlet/tarr?regser=serial&entry=85634522

The same was done for Dawnguard before it was announced- http://tarr.uspto.gov/servlet/tarr?regser=serial&entry=85546517

To see what this potential DLC could entail, Bethesda forum user Phyber pointed out the events during the month Hearthfire:

Heartfire is the ninth of the 12 seasons in the Elder Scrolls universe. It is considered to be a Fall (Autumn) month. (by our definition).
Annual Events during Hearthfire:

• 1st of Hearthfire: The Rites of Moawita practiced by the Psijic Order.
• 3rd of Hearthfire: Tales and Tallows - A celebration of Necromancy. Many citizens fear this day, and avoid the streets.
• 6th of Hearthfire: Khurat is celebrated in the Wrothgarian Mountains
• 12th of Hearthfire: Riglametha held in Lainlyn.
• 19th of Hearthfire: Children’s Day celebrated in Betony.

Seems like either the DLC could entail something with the Psijic Order, magic, necromancy, and/or High Rock. Or the trademark could be a trick on the marketing front since they could know people look up their trademarks like they did in the past for Skyrim, Oblivion’s DLCs and expansion, Oblivion, and I think for Morrowind and it’s DLCs/expansions too; Another thing to note is that Hearthfire is September in the Elder Scrolls universe http://www.imperial-library.info/content/calendar-tamriel, so maybe that’s when the DLC could be announced or released, a food for thought.