I can't explain why this is a new behaviour, but it sounds as though he has done no more than many dogs do, and truthfully I wouldn't call him an aggressive dog, although I understand why you want to make sure things don't escalate.
Many adult dogs have a tolerance limit towards pups (and other dogs) that rush up to them and are pushy. In dog body language, it's a very rude and impolite way to greet another dog and it makes many dogs nervous. Alfie warned the pup with a growl and when the pup ignored it he pinned him down. That's fairly normal dog behaviour and wasn't an attack or overtly aggressive. Unpleasant for the pup, pup's owner and you, but it was not an act of unprovoked aggression. One of my two will do it to pushy pups because he becomes very anxious, and because of that I put him back on lead when I see a pup approach and call out to the pup owner asking them to do the same.
Dogs on lead are often worried about meeting others. Being on lead makes it difficult for them to display their normal body language and difficult for them to read the restricted body language of the other dog on lead. That can make one or both become reactive and ready to growl or lunge as a reaction. Again, it doesn't mean the dog is showing unprovoked aggression. If I'm in that situation I keep away from other dogs on leads by changing course or crossing the road or whatever, with a cheerful and positive "let's go" to my dog to help him keep calm and not get anxious. It can be a nuisance to do, but many other dog owners understand that dogs on leads are usually on lead for a reason and are kind enough to take a diversion themselves.
Can I suggest that you do some reading on canine body language, which is a fascinating subject, and also consider finding a trainer who uses modern, positive training methods, to help you understand why Alfie is reacting and to help you find a way forward.
