My dog hates other dogs sniffing his bum – why is this?

Hi everyone,

My dog is almost 18 months old. He is usually very sociable, but lately his behaviour towards other males has become quite worrying. As soon as a dog approaches and tries to sniff his rear end, he hates it and snaps at them – even if the other dog is only a 2 or 3-month-old puppy. Why is he doing this? Could he have been traumatised as a puppy? Where we used to walk him, there was a dog that tried to hump every other dog, so maybe he was traumatised and it's all coming back to him now. What can I do to stop him from snapping at other dogs when they do this?

Thanks!