WMG / Colossal

Cause of the Kaijus is...

Regardless of the Lightning Can Do Anything primary cause of the driving aspects of the plot, why Gloria and Oscar have their kaiju can perhaps be explained by quantum entanglement: the lightning strike in the original incident gave both of them the ability to quantum-tunnel to their respective toys. The toys somehow grew to kaiju-level size in the 'other dimension' and were subconsciously transported via the quantum tunnel to Korea, which might have been on Gloria's mind (as Korea was the subject of the diorama that Oscar destroyed). Now, everyday at a certain time, the quantum tunnel opens again to both of them if they're in the vicinity of where the original incident happened, and they're thus able to entangle their consciousnesses with their kaiju counterparts in Korea and the hijinks ensue.