There are a couple of theories on why President Trump revoked the security clearance of former CIA Director John Brennan.
Two White House officials told The Washington Post that Trump viscerally hates Brennan and believes targeting him made the president look strong and decisive, adding that Trump has the paperwork ready to hit other officials tied to the Trump-Russia investigation. Four sources close to Trump told Axios that Trump discovered, thanks to Sen. Rand Paul (R-Ky.), that he could revoke security clearances unilaterally, like issuing pardons and signing executives orders, and he's enjoying the thrill and instant gratification of another absolute power.
Another common theory is that Trump is trying to silence dissent within the intelligence community. But Trump's Twitter feed bolsters a theory by Eli Lake, who argued in Bloomberg that "far from trying to silence Brennan, Trump is elevating him. He wants to make Brennan the face of the so-called resistance. This is the Trump playbook. Why do you think he keeps tweeting about Maxine Waters?" Trump views Brennan as "a perfect adversary," Lake says, because he distracts from Trump's other scandals, represents the so-called "deep state" Trump rails against, and makes "an easy political target."
Trump has been tweeting about Brennan all weekend and throughout Monday, but since he moved against him last week, basically the entire intelligence and foreign policy community not currently working for Trump has publicly criticized the president and his politicization of national security. The first dissent was from retired Adm. William McRaven, who oversaw the raid that killed Osama bin Laden, and then more than a dozen CIA chiefs and deputy chiefs from every administration dating back to Ronald Reagan's signed their own statement, followed by 60 prominent former CIA analysts and officials, and 177 other former U.S. national security and foreign policy officials released their own letter on Monday. You can read all their names and titles at The Washington Post. Peter Weber