The people perpetrating them are copycats of the Russian racist groups, which have been doing the same thing in Russia for years. Their racist messages in Ukraine and the graffitti they leave behind are even written in Russian, and even in Central and Western Ukraine, which are Ukrainian-speaking regions. For example, in this racist graffitti from Vynnytsia, the message ("Kill the Negro") is written in Russian, even though this area is actually West-Center of Ukraine. If this was really the work of Ukrainian nationalists, you can bet they would have used Ukrainian language.
As the article correctly notes:
"Ukrainian hate groups are believed to be inspired by their counterparts in Russia, where minorities are assaulted almost every day. Russian skinheads help the local groups, sharing tips and video clips on how to attack and torture their victims and how to safely leave the crime scene, rights groups say."