Good questions, especially when you compare S-F vs Monster era :
TOS #1 (January 1959) - #4 (July 1959) : S-F
TTA # 1 (January 1959) - #4 (July 1959) : S-F
ST # 67 (February 1959) - #69 (June 1959) : S-F
Then starting in August/September 1959 the monsters start in the three titles
Could it be linked to some movie (or movie genre) that became popular in the summer of 1959 ?
Searching on the web I found the following movies from year 1959 :"Giant Gila Monster", "Behemoth, the Sea Monster", "Gigantis, the Fire Monster" and "Monsters of Piedra Blanca" that have the same type of monsters than the one seen in ST, TOS and TTA from 1959 on...
Before 1959, big monsters movies were more S-F type. Maybe that's where Stan Lee got his inspiration.
Great research guys!! Those dates aren't coincidence. My personal opinion was that these titles and JIM pre-hero spoke about the times and pop culture and you can really pick up on the evolution of the stories based on popular themes for the days. As Strange Tales got on in the later years. I believe the effect was the same with the Nick Fury -- SHIELD stuff as mid to late sixties culture picked up on the spy and espionage theme ala.. James Bond