When you turn it on, all 18 servos try to go to the default position at the same time, how many amps does that require? Try attaching the servos 2 or 3 at a time with a, say, 75 mS delay between groups.
The buck converter is rated up to 20A
Sounds like a marketer's brag, for how long, 5 milliSeconds? What is the buck's continous rating?
And post a wiring diagram.