It is psychology. You are in a relaxed frame of mind, meditating on the programme content - sometimes we arent actually taking in the programme (unless it is a programme we are actually interested and enthrauled by), we might be in our own world of thought. The boom of the ads is a wake up call - 'look at me! Im exciting! I want your attention!'
Once the ad has your attention, it might actually have a chance to sell you something. Subconscious 'blip verts' - singlar flash ads designed to enter your subconscious without your conscious thought noticing - are not allowed in advertising, so 'in your face' ads are the new desire.