In Animal Farm, why did Napoleon seem to feel threatened by Boxer's death?

In Chapter Nine of Animal Farm, Boxer injures himself while collecting stone for the completion of the windmill. When word of this accident reaches Napoleon, he has Squealer, his propagandist, tell the animals that he is arranging for Boxer to be treated by a local vet. But the truth is quite the opposite: Napoleon sells Boxer to a horse slaughterer and pockets the money for himself and the other pigs.

When the van comes...

In Chapter Nine of Animal Farm, Boxer injures himself while collecting stone for the completion of the windmill. When word of this accident reaches Napoleon, he has Squealer, his propagandist, tell the animals that he is arranging for Boxer to be treated by a local vet. But the truth is quite the opposite: Napoleon sells Boxer to a horse slaughterer and pockets the money for himself and the other pigs.

When the van comes to collect Boxer, the other animals gather to say goodbye and quickly realise what Napoleon has done. This is a dangerous moment for Napoleon which threatens his position as leader, because every animal on the farm loves Boxer and they are horrified to learn that he has been sent to "the knackers." Napoleon's reaction demonstrates his sense of fear. Through Squealer, he lies to the animals by saying that the vet has recently bought the horse slaughterer's van and has not had time to paint over its sign. Squealer also claims to have been at Boxer's bedside at the time of his death and that his last words were: "Long live Comrade Napoleon! Napoleon is always right." This sounds like something that Boxer may have said, as one of the most loyal and dedicated animals on the farm, and so the others believe Napoleon and Squealer's lies. 

This acceptance of lies removes the threat to Napoleon's leadership. But when the other animals see the pigs drinking whiskey, rumours begin to circulate about where Napoleon got the money from. This episode, then, marks the beginning of the animals' realisation about Napoleon, specifically his tyrannical nature and his plans for the future of Animal Farm.