Lightning Dust: Skychaser!
 
Skychaser: ….
 
Lightning Dust: You will NOT ignore me in my own household, boy-!
 
Skychaser: Oh, so you want to control whether I want to talk or not too? Great. Fascinating, mom.
 
Lightning Dust: growls I’m not the one throwing his life away by lazing around and destroying everything he’s ever built! Refusing your training, wandering off to Celestia knows where-
 
Skychaser: narrows his eyes and scowls over his shoulder Training YOU forced me into. Remember? The whole “live out the dream I never got to” thing? The “let me insult all the things you’re actually interested in and throw some absurd training at you so you can pursue the thing I couldn’t even achieve myself” thing??  
Or did you conveniently forget?
 
____________So originally I was gonna keep it a mystery but OH WELL
 
Skychaser’s mom is Lightning Dust, who from the very beginning had envisioned a future where her son would become a Wonderbolt; the very thing she failed to accomplish. Despite Skychaser being more creative than sporty, Lightining did what she could to discourage Sky’s other interests in order to get him focused on the path of a talented flier. She started training him at a young age and was very hard during her training, giving him rather ridiculous training regimes for a young colt and using harsh words to “encourage” him to do better. Sky grew to dread training, but went along with it with the insistence of his mother.
 
After years of this, along with being criticized for seemingly every little thing, Sky began to loathe the training and harshness of his parents, growing angry/frustrated, distant and rebellious. He started snapping back, being sarcastic, and skipping training in favor of leaving his suffocating home for time to himself. His parents simply assumed this was due to him becoming a “moody, dramatic teen” instead of actually trying to understand his feelings.
 
And the more Lightning pushed, the more Sky rebelled and became cold and indignant to everything.
 
Eventually this all leads to Lightning Dust disowning Sky due to the problems he brought in (and possibly out; still planning) of the household.
 
One idea is that an argument over Sky’s future got so bad that Lightning decided his lack of direction in his life, the embarrassment he became, and the constant fights and negativity he brought to the household made him no longer have a place there.  
But we’ll see yo.
 
Feat:  
Skychaser