Steve:
You had a new record you wanted to share with Steve and thought it was the perfect music to dance to.
Bucky:
You were both feeling really stressed and needed to blow off steam, so of course a wild dance party ensued. It involved quite a bit of alcohol and definitely led to some other ways of stress relieving as well ;)
Pietro:
He had WAY too much energy and you wanted to get rid of some of it so that you could get some sleep.
Loki:
You wanted to learn a few dances that Loki learned in the palace growing up and he was eager to teach you.
Clint:
Clint got very drunk and started a dance party in the living room and begged you to join when you got home.
Thor:
You were cleaning and started dancing to the music you had on, when Thor came home and decided to join you.
Tony:
He decided to get back at you for all the times you interrupted him with AC/DC in the lab, and came to blast it in the living room where you were sorting paperwork.
Natasha:
You had a really bad day and needed cheering up, so Nat blasted some really bad 90's music and moved the coffee table out of the way.
Wanda:
Living room dance sessions are how you two celebrate achievements that either one of you have made.