PSV Horizonal Discharge to Atmosphere of Flammable

I'm searching for information on the origin, basis, reasons, etc., for discharge piping from pressure relief valves in flammable vapor service to be oriented horizontally rather than vertically. Although my read of API 521 leads me to conclude these systems need to discharged vertically, I've seen many examples in my career where these discharge horizontally. See link to CSB information on an incident resulting from horizontal discharge of ethylene from a relief system.

www.csb.gov/assets/1/20/csb_kuraray_factual_eng06.pdf?16387