Seems Soarin likes the smell of poor Dashie’s squishy padding~
 
Possible rough farfetched backstory:  
Dash is diapered because of a stunt she had to pull, I already explained in a different story how pegasi would need diapers in certain stunts.  
Dash wets after her stunt, she still needs to pee. She removes her flight suit and finds herself in a wet diaper in the locker room, squirming, near Soarin, who is there, lost in his own thoughts.  
Soarin notices the smell.  
“POMF!”  
Spitfire goes there to check on Dash’s performance during the stunt, and sees the scene. She’s jealous.  
Spitfire gets diapered.  
Spitfire shows her diapered butt to Soarin and wets to arouse Soarin as well.  
Spitfire teases Soarin: asks him to check “this soggy filly’s” diaper, wiggles her soggy rump in front of her face…  
Dash is shocked.  
Dash wets herself in shock, she doesn’t believe what’s happening.  
Soarin turns his head to Dash, shocked as well.  
Spitfire looks even more jealous and squishes her diaper against Soarin’s groin.  
Everything goes kinkier and with more heat.  
Somehow Soarin ends up diapered as well.  
Whatever.