The vote was on the Campbell Case but the primary reason was the Russian Treaties:

"Baldwin was careful to emphasise that the Treaties were the substance of his objections. Standfordham wrote to the King, on 7 October, 'I saw Mr Baldwin who, in reply to my question "Do you want to turn the Government out?" said "Yes, but not on the Campbell issue, but on the Russian Treaties which the country generally condemns." He sees no alternative to a dissolution; it has always been expected, some wished for it before now, though he is opposed to hastening the Government's fall. ...' ... And it is true that only in the House of Commons were Baldwin's strictures based on the Campbell case – at Newcastle on 2 October he congratulated MacDonald on the agreement, achieved at a conference in London, on the Dawes Plan for reparations, and then spent the rest of the speech alternately pouring scorn on the Labour party's subservience to its extremists, and tearing to pieces the Russian agreement. But the final debate on 8 October was a mean Parliamentary occasion. Two days before, in a meeting which was something of a suave qui puet, the Cabinet decided to take it as a vote of confidence, preferring to go out on the Campbell case to a defeat on the Russian Treaties which might give the left wing too much prominence.—Middlemas and Barnes, p. 273.