Way too much pure oxygen in that capsule was the main problem if I remember correctly.
The capsule being covered in Velcro, which was highly flammable, played a role as well.
Iirc there was supposed to be an engineer in the capsule with them. Unsuited, in the void storage space behind the seats. He ended up staying outside, but was convinced that he could have snuffed out the spark (possibly with a portable fire extinguisher kept in the cabin) right away and avoided the tragedy.
“From the Earth to the Moon” was excellent, as was Discovery Channel’s “When We Left Earth: The NASA Missions”