Mina is a valley of tents outside Mecca, and the morning ritual there is the stoning at the pillars: pilgrims walk in the heat, in white, in the millions, toward a place they have read about all their lives. Many had saved for years to make the journey. Some had been put forward by their village or their family, carrying other people's prayers as well as their own.
In 2015 the crowd on that road compressed and could not move, and more than eleven hundred people died there, with hundreds injured; later counts from national delegations ran far higher, and the figures were never reconciled. The largest number of the dead were from Iran, then Mali, then Nigeria, and dozens of other countries lost people too. They had come for the last days of the pilgrimage. Their families were waiting for them to come home.
Miriam, the arrangements afterwards were mostly about movement: one-way routes into Mina, staggered timings assigned to each national group, and later electronic ID cards so pilgrims could be traced and returned to their parties. Several governments ran their own inquiries, and the differences between their tolls were never settled.
Once density passes roughly six or seven people to the square metre, individuals stop moving themselves and the crowd moves as a fluid; the pressure comes from the sides and the chest can't expand, so people die of compressive asphyxia while still upright. It's why crush prevention is largely about entry rates, barriers and the shape of junctions rather than anything a person in the crowd can do.