It’s still “alive”, so he has to chop it one or several times more to “kill” it. Imagine that the tree is a fish, it’s common to smash one on the head and think it’s dead, then it starts wiggling around so you need to smash it again.

After falling, the tree trunk was still connected to the stump and was starting to rise again.

Only when the lumberjack chopped the remaining bit did the trunk fall for good.