Any androgen used in large enough quantities as to elicit muscle growth will cause some suppression of the HPTA axis. When this happens, your endocrine system is no longer producing the androgens it normally does, and as you probably know, these androgens are what allows you to gain/mantain muscle mass and strength. After discontinuing AAS there's the period usually called "post cycle" where your endogenous hormone levels will (hopefully) return to baseline levels. Untill they do, and this process can take quite some time in some cases, you'll be in a very disadvantagous position in what concerns muscle mass/strength mantenance. Post cycle therapy auxiliar drugs usually speed the process up and avoiding some androgens will help minimizing this recovery period.
In layman terms I think this is pretty much it.
Bryan also has noted that when people take advantage of strength gains and use weights beyond what they can use naturally, they are usually forced to lower them in the post cycle period. This, coupled with HPTA supression, can lead to losing most of your gains since the repeated boot effect resulting from the heavy loads used while on androgens will prevent the ocurrence of the microtrauma natural trainers are dependant on in order to elicit muscle growth.
Reducing calories while post cycle doesn't help either.