Eb was always short tempered, it helped power his magic. Eb was training Maggie Snr as his replacement as Blackstaff, she ran away and retirement from that role had to be delayed, he was then affected by Peabody whose influence was to the nudge Senior Council to be more like themselves, the Merlin more cautious, Eb even more shorter tempered, and subject to his prejudices, subject to the Blackstaff’s influence and older, suffering from the frailties of old age including Dementia. It all accumulates.
Then there is the Hubris. He has pretty much won every battle at this point otherwise he would be dead, he knows best, his continued existence is proof of it and he can break the Laws of Magic with impunity. Harry beating him is as bad a shock as finding out Thomas is his grandson.