I have Silver status on EVA's now renamed FF program. I've been automatically upgraded cabins from Elite to Royal/Premium Laurel about a third of the time on the SFO-TPE/TPE-SFO route.

Just wondering, other than the fact that Elite class was oversold for that particular flight and they needed to clear my seat for a non-tier status passenger, what criteria do they use to grant me a free upgrade? Traveling alone seems to be part of it - what else? Arriving at the airport early?