When you like a video on YouTube, the post will be added to Tumblr using the YouTube embed code. This is to make sure Tumblr doesn't try to "upload" the video for the Post.
Then
Create a video post
This Action will create a video post on your Tumblr blog from the given URL to a YouTube video, a Vimeo video, a URL to a video file, or an embed code.
Explore more great ways to automate Tumblr (legacy) and YouTube