According to ESPN.com, Stewart was required to be weighed so NASCAR officials would be able to determine whether the weight of his car met the standard measurements. After failing to do so, he went ahead and began his practice session anyway.
That resulted in NASCAR pulling him off the track and into the hauler for a short talk. After that, Stewart acquiesced. (Bleacher Report)
Tony Stewart on NASCAR black flag: "Go f---ing ask them. Go ask those f---ing rocket scientists why they f---ing black flagged us."— Jordan Bianchi (@Jordan_Bianchi) February 13, 2015