Here are some ways to fix it:
Option 1: Add a subject and verb.
* "Most of the movies I like have a lot of explosions, sometimes horror though."
Option 2: Combine it with another sentence to create a complete thought.
* "Most of the movies I like have a lot of explosions. Sometimes, though, I prefer horror."
Option 3: Add a coordinating conjunction to connect it to the previous sentence.
* "I love action movies. Most of the ones I like have a lot of explosions, sometimes horror though."