I don't find anything disrespectful in particular. When someone beats me quickly and disconnects, my feelings are hurt for a little bit, but then I get over it. I'm probably not fun to play if you're any good. Maybe they were interrupted and had to leave quickly. Maybe it's late where they live and they promised themselves this was the last round of online Smash they'd play. Maybe they just didn't want to play anymore. God knows I get tired of playing.
Also, when someone wipes the floor with me, I'll probably leave. This isn't some mentor-student relationship where they're showing me their ways and I must study them; it's a game and if neither of us are likely to enjoy it, I see no reason to keep playing with you. I'll never be good at Smash - I've accepted that and moved on.
If a match was close and fun, I'll probably stay. If they leave, I'll be disappointed, but once again, life happens. Smash is what I do to relax and chill after a tough day.
I'm wondering how many people I've unintentionally disrespected online. "freaking yoshi players with their egg spamming and forward smashes ugh so stupid and they ragequit too wow cry moar" Rest assured, I have no intention of disrespecting people, but I'm vaguely amused at the prospect of someone interpreting my actions as disrespectful.