Chainsaw chains come off the bar due to worn-out bars, worn-out sprockets, and wrongly set tension. You should also note that with continued use, chains lose their tension and hence are more likely to come off the bar when you least expect it.
Why does the chain on my chainsaw keep coming loose?
Chain Not Set Properly
One of the most common reasons why your chain might be coming loose is because it was not set properly. When you go through the steps to tension your chain, you need to finish those steps by setting the tension so it won’t change as you work your saw.
Why won’t my chain stay on my chainsaw?
Quote from Youtube video: If it's worn down in shallow you should replace the bar immediately inspect the chain for any worn bent or broken teeth. And replace it if necessary.
