The acquisitions began indeed in 1990. According to Jeff Tunnell:
“I founded Dynamix in Eugene, Oregon in 1984, guided the company from an independent game developer for Electronic Arts to an affiliated label of Activision, then sold the company to Sierra On-Line, a publicly traded company (and now the entertainment division of Vivendi/Universal), in 1990.”
http://www.armchairempire.com/Interviews/jeff-tunnell-garagegames.htm
It’s off-topic, but I wondered about another issue, the acquisition of Broderbund a year later? Was this indeed the case?
See this article
http://www.findarticles.com/p/articles/mi_m0NEW/is_1991_March_11/ai_10455931