Increase in Aggression

[AimeeA]

I have had my dog Brady for about a year and he is roughly two years old, the vet guesses he is a mix of a lab/Weimaraner/beagle. I have only seen him bark and growl at one person and that was a cable repair man who came into the house unannounced. Recently my land lord came to show the house to new tenants with out warning, when she tried to enter my room while I was asleep (thankfully the door was locked) Brady got very aggressive barking and growling until I got him in his kennel and let him know everything was ok. After that incident he had gotten very protective barking when someone enters or leaves the house when he can't see them, barking and growling (no bearing of teeth or hair standing with these growls) when someone knocks on the front door or is outside my bedroom that he can't see, he also has started to bark at random people walking down the street (which I have nerve seen him do). We also had one incident when a local homeless woman came into our yard to try and pet my roommates new puppy, Brady ran across the yard growling and hair up. I am just curious as to why he could be having these sudden burst of aggression and if this is a behavior I need to be concerned about and correct immediately. I don't live in the safest area so I like that I have him to be alert but I don't want him to ever be in a situation to bite and innocent bystander. He has never bitten anyone and relaxes as soon as I tell him to calm down and it's ok.