It's thought that "sust flu" is a reaction to the unusually high androgen levls that have just been introduced to the body. The body isn't used to this hormonal environment and as such activates defense mechanisms agains this oddity...much like and infection. It doesn't just happen with sust. All AAS can cause this, however the rapidly released proionate ester in sust quickly elevates blood testosterone levels and the body recognizes this and acts in defense. Hormones are extremely potent molecules and modulate much of the body's physiology. An alteration in the "normal" hormonal status can have profound effects on the body as is seen by the increase in muscle mass, but also by the uncomfortable symptoms accompanying high doses of AAS. Everyone reacts differently and every anabloic steroid molecule acts differently within the body which accounts for the discrepencies in personal accounts of "sust flu", but it does exist and some people react with greater magnitude.
The insomnia is likely due to central nervous system stimulation that acoompanies AAS use. Steroids stimulate the nervous system as well and individual myocytes and cause and excitory reaction. Many androgens also increase the population of beta-adrenergic receptors which will also intensify a state of alertness and even edgyness as catecholemines are binding in higher concentrations than previously was possible.