Why are only some aircraft in this formation creating contrails?
This 'party trick' is covered nicely on metabunk.org.


  [It's the Su-30's] ability to inject unburnt fuel into the exhaust plume via the afterburner system. The super hot air of the exhaust instantly evaporates the volatile fuel, and then when it mixes with the cold air behind the plane the fuel reforms as a fine mist, essentially a fuel smoke cloud.


(YouTube)