It seems like around 1200-1000BC, such a wide range of cultures moved into the Iron Age, from the Greeks to Mesopotamians to Indians and I've even read of iron smelting being used in Sub-Saharan civilizations (such as the Nok) starting around the time. I've heard of the Bronze Age collapse possibly caused by the mysterious "Sea Peoples", but that wouldn't explain Vedic civilizations using iron.
To me this points to some global weather pattern/geological shift that made iron more available and logical to use or made tin less available. Is there any evidence of the sort? Or is it all one massive unsolved coincidence?
The process actually took a much longer time. We have evidence for iron being used deep into the 2nd milennium BC, but it remained a rarity / luxury for centuries. Perhaps this is because its manufacture and full potential was still poorly understood, or perhaps because - as a material - it lends itself to very different scientific / cosmological / magical ways or relating to the world than bronze does (cf. Chris Gosden 2012).
Iron starts to really catch on in the Near East around the beginning of the first milennium. Around 1100, the Mediterranean was experiencing a severe economic and demographic collapse, almost certainly as a result of climate pressures that led to crop failures and the disappearance of most major Mediterranean civilizations. A popular theory argues that this 'Dark Age' cut off access to the long-distance trade routes through which copper and tin reached the Near East, forcing people to find an alternative metal. And iron ore, unlike copper, is plentiful in most parts of the world - once bronze was cut off, people realized iron's advantages and never looked back.
It's also possible that the shift to iron accompanied transformations in religion and understandings of the world - again, see Gosden 2012. His article, though brief, makes the important argument that radical changes in technology usually involve social transformations as well as economic pressures, and he notes that many cultures seem to have resisted the adoption of iron long after the starting date we usually assign to the iron age, he inks because bronze was so central to their magical understanding of how technology ought to work.
Iron doesn't really become common in Western Europe until a few centuries later still (c8th century), once population is recovering from the 11th century collapse.
So it's not a sudden transformation - iron took roughly a millennia from its discovery to replace bronze as the default material for tools.
But some changes did clustered around 1200-1000 BC, and these occurred during the massive social upheavals that followed a period of climate change, famine, and economic and social collapse / slow recovery.
Gosden, C., (2012), Material, Magic and Matter: understanding different ontologies: in “Maran, J. and Stockhammer, P. (eds) Materiality and Social Practice. Transformative Capacities of Intercultural Encounters”, pp 13-19, Oxford: Oxbow Books.