iDiskk, LLC of Campbell, California, simply threw the lithium ion batteries in a dumpster with regular garbage. In three separate incidents, when the dumpsters were picked up by garbage trucks and compacted on board, they ruptured and caught fire.
In one incident, which took place on October 13, 2021, the garbage truck was forced to eject their entire load in the middle of a residential street to prevent the vehicle from catching fire. According to the Santa Clara County District Attorney's Office, in the process of jettisoning the smoldering trash, the truck's lift arm also felled power lines, which required repair.
The other two fires from iDiskk's improper battery disposal occurred on September 22 and October 6 of 2021. The DA's office says that the company cooperated with their investigation, took steps to improve compliance, paid the civil penalties, and "agreed to be bound by a permanent injunction never to improperly dispose of hazardous waste again."
Lithium-ion batteries can explode when the separators between the negatively charged anode and positively charged cathode break down, causing a short circuit. These short circuits can result in a runaway thermal reaction. The same thing that caused fires in the post-Hurricane Ian flooded Teslas also caused HP laptops to be recalled in 2019 and Samsung Note 7 phones to be recalled in 2016. In an attempt to make batteries more compact, separators have become very thin, thinner than 20 microns is some cases. It's easy to see how they wouldn't survive the 2,750 psi compactor found in a regular garbage truck.
Source: Autoblog