Avocados are popular in California and the US West Coast received the majority of Japanese immigration for a long time, so you have an opportunity for cultural cross-pollination. The famous "California Roll" contains avocado and was invented in Los Angeles, so you can see how Japanese cuisine would come in contact with avocado. (See sidebar "California Roll's origin is traced" in the below link to the San Diego Union Tribune.)

http://www.sandiegouniontribune.com/uniontrib/20050824/news_lz1f24sushi.html