There are published reports of well connected, powerful people with official information who passed it on in closed door (but secretly recorded) meetings to their um, friends. Go search if you want to know more. I didn't see Iger mentioned by name in connection with this but I also think the timing of his departure was ... fairly remarkable.
This doesn't always work against Disney shareholders' and employees' interests. For example Disney offering a $6B bond issue just one day before the Federal Reserve announced it would begin buying corporate bonds for the first time in history sounds a lot like someone got another, timely "special briefing". Or it was another amazing coincidence.
The counter argument to the above is that the same basic information was available publicly and that people like Iger who apparently had magical timing were only benefiting from common sense and good luck.
BTW selling shares by company insiders must be preceded by an announcement of some kind for example, "CEO Joe Smith announces that he may sell up to X,000,000 shares in the next Y months." After the shares are sold I believe that the exact number sold (if any) and date must be announced. There may be some delay but I think that anything significant that Iger sold can be looked up at some point.