Phil Collins composed much of his first solo album - from which 'In The Air Tonight' is taken - as therapy for his very painful first divorce. This explains a lot of the anger / pathos themes that run through the material. As far as remember, "Sussudio" is just a nonsense word - Collins often looks for lyrical angles by "Lah lah" ing and singing meaningless sounds to the music, until something comes into his head that can be worked up into a lyric, or in this case, left in because he liked trhe sound of it.