[Amps] Why does power output increase?

[Victor Rosenthal 4X6GP]

This isn't a problem, just a phenomenon that I would like to understand.
I have a TL922. Mods include directly grounded grids. I also measured 
the filament voltage with my Fluke true RMS meter and put a small 
resistor in the primary of the filament transformer to provide exactly 
5.0v in standby.

It has been running with the same genuine Eimac 3-500Zs in it for about 
6 years.

So here is what happens: after a longish CW transmission, the power 
output INCREASES. On 20m, for example, it may go from 1200 to 1400 
watts. When it cools, the power drops back.

I've noticed this with several wattmeters. I have observed no change in 
SWR when this happens (it's 1:1 thanks to a Johnson KW Matchbox).

It happens when the amp is driven with an Elecraft K3 and also a TS890s.

I'm interested in hearing theories about why this might occur.
-- 
73,
Victor, 4X6GP
Rehovot, Israel
CWops #5
Formerly K2VCO
https://www.qsl.net/k2vco/