About 90 seconds before the planned launch toward the orbital station Salyut 7, a fuel leak occurred at the base of the Soyuz-U rocket, caused by a fault in a fueling valve. The leaking kerosene ignited almost immediately, and a fierce fire broke out on the launch pad.
The crew — commander Vladimir Titov and flight engineer Gennadi Strekalov — were already strapped into the Soyuz cabin atop the rocket and at first had no knowledge of what was happening on the pad below them.
The automatic system that was supposed to immediately activate the escape system in the event of a fire failed — the flames burned through the wiring before the signal could reach the launch mechanism.
Wikipedia: Launch escape system · NASA History — Apollo — full list on Escape and Backup Systems in Spaceflight