What caused the bus fire?

In Wuxi, 24 workers died in a late-night steel company shuttle bus fire.  The bus was going through a Hui Shan Mountain tunnel when the fire started.  One student has told me of rumors that say the fire was caused by a rival Wuxi steel company.  Apparently, the rival company had sent thugs, to the other company's owners house, who beat up the owner and his wife.  The bus fire is the latest incident in that rivalry.  One company owner is very angry at losing market share to the rival is the motive for the incident, supposedly.

But it's a rumor --  I have only heard it from one person.