Why is my neutered dog suddenly humping and acting aggressive?

No, there is nothing wrong with the surgery. Dogs hump for other reasons like dominance behavior, play behavior or excitement. I suggest you refer to this article linked below: ( https://www.aspca.org/pet-care/dog-care/common-dog-behavior-issues/mounting-and-masturbation )

This is not a sexual behavior, but likely a response to stress, anxiety or to assert his dominance. Correct the behavior immediately when you see it. Remove him from the dog with a stern "No" and offer a toy or task to distract him. Praise and reward him when he complies. If he persists, separate him until he settles down. Be firm and be consistent.

Has anything changed in the household? New people? New pets? Babies who started crawling/walking? Construction? Guests? Change of furniture? Changed schedules of routine? Etc.. Sometimes when there is some type of change it can strike up this behavior because they are anxious and trying to figure out what to do with themselves. They can also do it if they are trying to portray dominance. You can tell him "no" and take his toy away. If there is a particular toy that he likes to hump, I would get rid of it. You can also give him a calming chew like composure and a calming collar like adaptil. I hope this helps!

This could be a dominance behavior or may be due to an abnormal production of hormones such as cushings disease. I recommend a full vet check and bloods especially if there are any additional symptoms such as altered appetite and or coat changes

The best thing you can do for him right now to help correct this behavior is to get him neutered. As he grows and starts to produce more testosterone, he can start to become more aggressive and dominant as male dogs are prone to these behaviors. Once they learn the behavior, it can be difficult to train them out of it. But if you spay him now before he has a chance to fully develop these tendencies, it will keep further behavior problems from worsening. It should also stop the humping behavior. After his neuter, if he still seems like he is having issues with aggression, you will want to enroll him in a training course or work with a behaviorist to help you establish yourself as the one in charge. Training courses are there to teach us how to respond to our pets as much as they are to train our pets to respond to us. Aggression is something you want to tackle early before they h ave a chance to become routine behaviors.

It's hard to say why Duke might be doing this with only one specific dog. Humping in dogs is often associated with frustration, over-excitement, or can be an attempt to solicit attention or play in some cases. It would help to be able to observe the dogs interacting in order to make a better guess at why he does this. If it's causing a problem, the best option would be to simply separate Duke from this dog when you are at your friend's house, or keep him on-leash so that he can't bother the other dog. You could also try giving him something special to chew on when he is there, such as a bully stick or rawhide, to help keep him busy.

Oliver is masturbating, which is a normal behavior in dogs. Even though he's neutered, he's likely continuing the behavior because he has learned that it feels good. If this behavior bothers you, I recommend trying to distract him from doing it. It's best to get his attention before he starts masturbating. If you see him starting to act like he is going to masturbate, throw him a toy, start playing with him, or ask him to perform an obedience skill (i.e. sit, stay, etc.).