Not fare mistakes but (my assumption is) if the automated fraud processing can't clear the reservation (usually functionally "instant") it gets routed to a desk for manual agent/internal security review before releasing it for ticketing.

Back in the period of time UA was leaking PNR comments to the wide world (RIP) you'd see the 'to fraud' and 'fraud automation returned Ok' comments basically on top of each other. It's been a long time since I've had a PNR not ticket instantly (native or partner, revenue or miles) let alone when the tool was showing comments so my sample size is zero but particularly for a pure UA ticket it would go a long way to explain the "random" delays