Basically, they saw a hardcore community form around their family friendly fighting game, and tried to axe it.
https://www.ssbwiki.com/Dash-dance#/media/File:Dashdance.gif
If you did this sequence in Brawl, you'd certainly trip.
Dash-dancing kept your momentum, allowing you to instantly move at full dash speed at a moment's notice. But it took great amounts of finger-dexterity to alternate left-right-left-right at this speed and keep the character's momentum at maximum.
If anything, it sounds like they took in some feedback from the community.