Sperling, 62, joined the company in 1996 and spent nine years as its chief executive. He is credited with building a real estate investment, development and management portfolio for the company such that it could exist beyond 2014, which is when the long-term Downtown Seattle real estate partnership that gave the company its start 60 years ago is set to expire.

Alfred Glancy, executive chairman of Unico Properties and Unico Investment Group, said in a prepared statement that Sperling's drive "gave the company life after 2014." Glancy chose not to be interviewed about why Sperling resigned and authorized a spokesperson to say only that the decision was "mutually agreed upon."

Reached at home on Bainbridge Island, Sperling tells GlobeSt.com that it has been "a terrific 13-year ride," that he's "enjoyed every minute of it," is "proud" of what he's accomplished in the way of portfolio growth, customer service, innovation and green buildings, and is "looking forward to new challenges." As for why he left, Sperling also declined to disclose the events and discussions that lead to the decision.