Apparently the new ruler wanted to appear liberal to other countries, so he locked up the activists before announcing that women would be allowed to drive. That way, they couldn't claim the victory of their years of lobbying and protests, and use the domestic and international media attention to argue for further issues. It was just him being modern.
www.theguardian.com/world/2019/mar/02/saudi-arabia-detained-womens-rights-activists-to-be-put-on-trial