In the passage below, why does "Squealer's demeanour suddenly [change]?"
"And at the end, almost too weak to speak, [Boxer] whispered in my ear that his sole sorrow was to have passed on before the windmill was finished. 'Forward, comrades!' he whispered. 'Forward in the name of the Rebellion. Long live Animal Farm! Long live Comrade Napoleon! Napoleon is always right.' Those were his very last words, comrades.' Here Squealer's demeanour suddenly changed. He fell silent for a moment, and his little eyes darted suspicious glances from side to side before he proceeded. It had come to his knowledge, he said, that a foolish and wicked rumour had been circulated at the time of Boxer's removal. Some of the animals had noticed that the van which took Boxer away was marked 'Horse Slaughterer,' and had actually jumped to the conclusion that Boxer was being sent to the knacker's. It was almost unbelievable, said Squealer, that any animal could be so stupid."