Why did Arkhenaten's reforms anger the priesthood?

Answer 1

Akhenaten was the first man to invent one all powerful god.
At the time in Egypt there were many gods and each had its own temple and priesthood who made a very good living from the offerings of the worshippers.
Akhenaten closed all these temples and just had the one god Aten and built a new city to his glory.
This put all the traditional priesthoods out of business and as soon as Akhenaten died they restored the old gods and destroyed his city.

Answer 2

Akhenaten went from polytheism to monotheism, putting a lot of priests out of work. So no wonder they were pissed!