A BBC Sound engineer explained this to me once, and it's due to the audio compression that is used when an adverts soundtrack is created.
It's nothing to do with compression as we understand it computer terms, but by compressing the audio signal you elevate the quiet parts of the soundtrack so they are at a similar volume level to the loud parts. This makes the sound much more punchy and full, which is what advertisers want.
What it means to you the listener is that adverts seem much louder. Although in effect they are not louder, there just is no quietness in them.
I hope that kind of makes sense...I tried to find you a link that explains it better, but Google just turns up loads of pages about digital compression, which is not the same thing.