When I reached lesson 24 of the standard tutor - EarMaster started playing a sequence of chords before playing the question.
From playing around with the menu options I am guessing that this is what's called 'Play tonic of key' - but it's really annoying and I wish I could switch it off. Is this possible?
That sequence of chords is a cadence, and as Hans said, it has a porpouse, and a very important one.
In tonal music (about 99% of the music most people listen to), a trained ear must identify intervals in the context of a key, and the cadence establishes that key.