It's because Mr Radley lied to Jem about the tree dying in order to sever the contact between Boo and Scout & Jem. When Atticus tells him that the tree's healthy, Mr Radley's true intentions behind cementing the hole dawn on Jem.
He's basically upset that they've lost the only connection they had to Boo. Plus, he won't be getting any more chewing gum presents. :)