https://www.youtube.com/watch?v=kjSFDAySllQ
WARNING: I had a hard time understanding this pattern. This entry is more questions than answers.
Billy of Go! Billy Korean told me to check out this video: https://www.youtube.com/watch?v=YLM-XL7LW5g
Our brief exchange in the comments section leads me to believe that this pattern is a combination of -어야 and [[-어서]]. Same thing with [[-야겠다]], apparently.