Could have been dropped in a bad way. This is the reason why Lithium Ion cells go bad, external damage. There are numerous examples of people falling with their e-bikes, not even visible damage on the outside of the bike or the pack other than maybe a scratched up handlebar and then 12 hours later the whole thing suddenly goes up in flames.
Here is one example caught on video:
https://www.youtube.com/watch?v=bbWmCiMJdT8