IIRC there were a bunch of contributing factors, starting with how the terrace was basically slats which allowed so much rubbish to accumulate under the stands, as well as the fateful dropped cigarette that fell through the gap and ignited the lot
There were also some other contributing factors, notably the stand's roof was covered in bitumen felt which would keep the rain off but was highly flammable, and there's eyewitness accounts of great globs of the stuff pouring down and encasing people as they're still sat in their seats. Plus the complete lack of consideration for fan safety, as the stand didn't have any fire extinguishers and the gates leading to the street were padlocked shut
There's also a certain amount of innuendo about Bradford's then-chairman, considering he had a track record of his businesses literally going up in smoke in the years before the fire