Why does my 23 lb cat become FIERCELY aggressive towards my 140 lb German Shepherd for no apparent reason?

I have 2 cats and a very big 5 yr old German Shepherd. He has been attacked "out of the blue" a # of times, by my large 23 lb (diabetic) cat. Dusty (my cat) on occasion has gone from being his sweet, loving self to acting like a possessed cat, with fierce aggression & attacks my dog, when Maverick (my GS dog) has done absolutely nothing to deserve the attack.  A few times he has actually even been asleep on his dog bed when Dusty has gone "ballistic" & has attacked Maverick (who is generally great w/ both cats, tho at times is not as gentle as he should be when he tries to groom them). It seems like it happens when there is maybe a quick move or a weird sudden unfamiliar sound made and I think it is related to this & it seems like he attacks Maverick because he perceives him as being the reason for this weird sound or quick movement & for some reason thinks Mav is being a threat to Boomer (Dusty's cat buddy-my other cat). Maverick is afraid of him now & I am so afraid of it happening again that I do not allow them to ever be physically near each other even. I have gotten in the middle of it each time, trying to protect my dog because he did not seem to protect himself when it happened! He was usually in a state of surprise &/or shock when it happened! What could possibly be the reason for this behavior in my cat? (He is on insulin by the way and for the most part is controlled blood sugar-wise)

[Jim Humphries, B.S., D.V.M.]

it is possible that the fluctuating blood sugar and age of the cat is causing him to not be himself, even aggressive.  Your best bet is to supervise them well or keep them apart as much as possible.