Why did Kazakhstan decide to secede from Russia in 1991. Didn't it have a plurality of Russians by that point?
Kazakhstan never seceded from "Russia." Kazakhstan was a member-state of the USSR, not of the Russian Federation. The Russian Federation was also a member-state of the USSR, the largest and most powerful member-state, but still merely a fraction of the whole. Not only that, but Kazakhstan never "chose to secede" from the USSR, either.
What happened, in summary, was that the leaders of the USSR's three Slavic states, Boris Yeltsin for Russia, Leonid Kravchuk for the Ukraine and Stanislau Shushkevich for Byelorussia (Belarus), formally
dissolved the USSR and replaced it with the Commonwealth of Independent States on December 8, 1991. This agreement was known as the
Belavezha Accords (despite the somewhat obvious fact that this was one accord, and not several

). This was entirely unconstitutional of course, but once it had been announced it was impossible for Gorbachev, the leader of the USSR, to stop the dissolution from taking place.
This, of course, is exactly what Boris Yeltsin, the leader of the Russian Federation and Gorbachev's primary political rival, had in mind when he organised the coup. Yeltsin had recently surpassed Gorbachev as the most powerful political figure in the USSR, after the former saved Gorbachev's arse - not to mention his own - by stopping the
August 1991 coup attempt, but with Gorbachev regrouping he needed to act to maintain his power. Dissolving the USSR effectively sidelined Gorbachev completely, as he was now the head of a state that no longer existed, whereas Yeltsin became a head-of-state himself overnight. Smooth move.
On December 12, 1991, Russia's withdrawal from the USSR was made official - if still not legal - when the Supreme Soviet of the Russian Soviet Federated Republic ratified the Belavezha Accords. On December 21, 1991, all the Soviet Republics except Georgia confirmed the Accords with the
Alma-Alta Protocol.
Kazakhstan declared its independence from the USSR on December 16, eight days after the Belavezha Accords and the day before it signed the
European Energy Charter Treaty as a sovereign state. Most of the former Soviet Republics signed this treaty of their own accord, ignoring the USSR's existence. Kazakhstan was actually the
last Soviet state to declare independence from the USSR - the Russian Federation never seceded, but is instead considered the successor state to the USSR. By the time Kazakhstan withdrew from the Union, in fact,
every single other member state had already done so. Most of them did so after the August coup, fearing that another would take place and hoping to keep themselves secure by removing themselves from the USSR, which doesn't make much sense considering the treatment of any member-state that had tried to declare independence before the coup; Lithuania's treatment was
especially shabby.
A quick check of
Kazakhstan's wiki shows that it has a plurality of Kazakhs (63.1%), not Russians (23.7%). There are no numbers for 1991, but the
census numbers in 1989 indicate that Kazakhs (39.7%) still outnumbered Russians (37.8%) though obviously not by nearly as much. It seems that much of change has been caused more by the much
higher birth-rate amongst Kazakhs than any active attempts to get rid of Russians, though many have emigrated to Russia.