My chronology may be a little fuzzy, but the basic gist is this:

During his legendary campaign, Alexander was in western Egypt at a temple and managed to get himself proclaimed son of Zeus or Ra or Osiris or some such. He then went on to lay waste to the Persian Empire under Darios. During the course of the Persian campaign, Alexander tried to integrate certain eastern (Persian/Indian) political ideologies and practices into his own rule. One of these was paying obeisance to the emperor by lying prostrate on the ground before him. For the Greeks and Macedonians, this action was only practiced in the worship of gods, so Alexander's Hellenic contingents vehemently opposed/resisted the notion, because it amounted to god-worship.

I believe that the basic thrust of all this was that Alexander assumed the position, in his own mind and conduct, of something of a demigod, and did his darnedest to spread the idea. Given that the Seleucid Empire is, essentially, the most easternly-influenced of the successor states of the diadochoi, it doesn't seem surprising that the general who came into possession of these territories would also espouse a relatively eastern concept of regal divinity.

Cheers.