There's not really a "specific reason" since he lost favour over some period of time, rather immediately in response to a single event. But some generally agreed factors were Stalin's paranoia and intolerance of dissent, as well as and Molotov's own personality.
Vyacheslav Molotov is well known to be stubborn and independent minded. He argued with Stalin over numerous policy differences, a behaviour for which he was prideful. We know this from both the observations of those close to them, as well as from Molotov's own remarks. For instance:
Some of Stalin's views I criticised ... and told him personally.
I am not the kind of person who was riveted by what Stalin said, I argued with him, I told him the truth!
It is no secret that Joseph Stalin was not one to take kindly to disagreements. Milovan Đilas, a prominent Yugoslav communist, observed that even in his old age:
[Stalin was] stubborn, sharp, suspicious whenever anyone disagreed with him.
- Djilas, Milovan. Conversations with Stalin. Houghton Mifflin Harcourt, 2014.
This is exacerbated by Molotov's extensive contact with the West and leading position within the Soviet Union. Until he was set aside and crushed, Molotov was a prominent member of the Soviet government and a seen as a promising candidate for Stalin's successor. Speculations that Molotov might be Stalin's successor fed directly into the Soviet leader's deranged paranoia.
The war had greatly aged the Kremlin leader, and foreign journalists began to speculate about Stalin's ill health and possible retirement. They even named Molotov and Zhukov as his successors. Reading press dispatches, Stalin began to suspect that his closest lieutenants (Beria, Malenkov, Molotov and Mikoyan) might no longer need his leadership and would not be averse to accommodating the United States and Great Britain behind his back.
- Zubok, Vladislav M. A Failed Empire: the Soviet Union in the Cold War from Stalin to Gorbachev. University of North Carolina Press, 2009.
Lastly, of course, there was also Molotov's loyalty to his wife, Polina Zhemchuzhina. She was the Commissar for Fishing in 1939, when the NKVD discovered a network of "German spies" in her commissariat. Molotov abstained from voting against his wife on that occasion, and repeated the same gesture again in 1948. The British historian Geoffrey Roberts, FRHistS, notes that"
Molotov's most damaging dispute with Stalin was personal and concerned Molotov's wife ... Molotov abstained in the vote on a resolution to expel her. No one else in Stalin's inner circle ever behaved in such a way. And it was not the first time Molotov had rebelled ... A decade on, however, Stalin was now overwhelmingly dominant in the party leadership and no scintilla of resistance to his rule was permitted.
- Roberts, Geoffrey. Molotov: Stalin's cold warrior. Potomac Books, Inc., 2011.