Getting air in the lines somehow

Very good point TB.
But IMO, I want to say highly unlikely. Brake fluid in the front lines moves only a tiny amount. Doing the math on a 2" piston I get 1cc per two pistons at a very large .010 piston retraction. In a 15/16 bore this is .177 inch or a little less than 3/16". At a more likely retraction of .005 say, it would be half that or about 1.4/16ths.
But since fresh Dot4 is in the entire system, there is really no other possibility.
Very good point TB..

Unless of course if the pads are dragging in the first place, then there could already be a ton of heat in the calipers, and dry Dot4 fluid is advertised to what? 446*F, IIRC. If braking starts at 200 say, well you get the picture.