Perhaps you're using some sort of master-bus compression? That is to say, a compressor that affects the entire mix? If so, when the vocals kick in, and if they are louder than the backing track, then the overall balance of the mix will be affected to some degree.
(I'm not sure what DAW you're using, but FL Studio defaults to having a limiter plug-in on the master bus, which is a type of aggressive compressor. If you're using FL Studio, remove the limiter and see what happens.)
If you're not using bus compression, then maybe you're simply experiencing a relative perception of volume change when the audio context changes...?