JR is right, you are swooping in the middle and its causing you to get hyzer even when your shoulder plane is parallel to the ground. And sometimes you are leaning into the throw quite a bit, which will cause even more hyzer (this is ok if you want
to throw hyzer):
To throw flat, keep your shoulder plane parallel to the ground and experiment with your pull line. Try pulling level at various heights (waist, stomach, chest). Experiment with more than just flat lines, also try starting out low and finishing at different heights. You need to exert some control over your arm while still keeping it loose, it's kind of flailing right now.
Slowing down the start of your pull will promote better timing, your acceleration is peaking a little too early. It also might make it easier to know what trajectory your arm is heading on.
You have the makings of good form, keep at it.