There were signs posted outside the cave to not enter during monsoon season. It is monsoon season. The coach (assistant) disregarded the warning.
https://en.m.wikipedia.org/wiki/Tham_Luang_cave_rescue
From your link:
Since part of the cave system is seasonally flooded, a sign advising against entering the caves during the rainy season (July–November) is posted at the entrance.[7]
They entered the cave in June.