Probably the temperature. At 25" MP and 2520 rpm, I'd guess you would
be running about 78% power under standard temperature conditions
(15C?). But at -1C the denser air is giving you additional power
output so you're probably closer to 80%. This is harder than most of
us would be willing to run at 5000 ft. Peak EGT at this power setting
is also asking for engine detonation.