When applesauce spills out of the jars during canning, this is a process called siphoning. It is just the contents of the jar overflowing. Not only does this cause you to lose good product but it also increases the likelihood of the jar not sealing - plus, it makes an unholy mess out of your canner! Because of this, it’s important to leave at least half an inch of headspace to accommodate for expansion. Also, the thinner your applesauce is, the less siphoning there seems to be. You can also reduce the likelihood of siphoning by taking the canner off the heat, removing the canner lid, and waiting before removing jars. This equalizes the internal temperature in the jar to reduce the likelihood of a mess.