I believe this is to do with spontaneous combustion, and I understand is usually associated with traditional fibres, eg cotton, linen etc., when they may be contaminated with organic-based ointments used to treat patients in hospitals in particular. If not fully washed out then the remains will start to oxidise. Increase the temperature and oxidation occurs more quickly. Thermally insulate the reaction by surrounding the contaminated laundry with other warm laundry and the reaction can accelerate leading to the spontaneous combustion.
With modern synthetics the problem is significantly less - they are less likely to trap deposits and will wash cleaner anyway.
A greater problem used to be laundry workers smoking and accidently or otherwise dropping the lit butt into a laundry basket; it's possible the new restrictions on smoking in the workplace may reduce the likelihood - or may not....!