Tail strikes are more likely to occur at high (density) altitude airports for several reasons — all related to density altitude. The same flare that works well at a sea level field like JFK can give you a tail strike at DEN. For the MD-11 Fed Ex that was likely loaded with cargo, the Vapp will usually approach Vls. Retarding the throttle on touchdown is done much less at high altitude airports than at sea level fields. You never really know how much the FMS software fully takes into account the high density altitude of the airport when computing Vref.
Because of the higher landing speeds and reduced effectiveness of the flare to reduce the sink rate at high altitude airports, the oleo-pneumatic shocks on the main gear will compress more on touch down at DEN than a sea level landing (assume all other factors equal). This Fed Ex MD-11 reported a "hard landing" which likely compressed their main shocks to near their full stop. That same landing speed and flare at sea level airport would have cleared their tail because it would have arrested the high sink rate.
Whenever flying into or out of a high altitude airport, it is best to get your sink rate under control earlier than normal and keep your speed up.
Choosing a higher flaps setting on takeoff will actually make a tail strike less likely because it gives you added pitch though clean-up is bit of a hustle. But on landing, pilots use the same landing flaps as at sea level airports (max), which means they will have less tail clearance than that same landing flare done at a sea level airport (again, all other things equal - i.e. plane weight, headwinds, CG, etc.).
To reduce tail strikes, Fed Ex should consider loading their planes with their CG moved forward, so after fuel burn they have less alpha on the flare.