What explains the change in the soviet stockpile from 1975 to 1985?

The Soviets were responding to a tougher stance taken by US President Ronald Reagan, including his proposal of the Strategic Defense Initiative (SDI) in 1983.

SDI, a proposed space-based missile defense system, was not the only measure taken by the Reagan administration to beef up American military preparedness in the Cold War.  When Ronald Reagan came into the office of President in January, 1981, he quickly moved away from the former policies of detente to what became known as the Reagan Doctrine.  The Reagan Doctrine involved many policies targeted at reducing Soviet influence in the world and ending the Cold War by the projection of US power.  In his first year in office (1981), Reagan announced his Strategic Modernization Program to ramp up and improve American military systems overall, plans to spend over 200 billion dollars over five years on bombers, cruise missiles, intercontinental ballistic missiles, and more.  So the USSR's weapons increases were, from their point of view, matching American plans to enhance military arsenals.