Where did that come from?
Just like humans, some days you can cope with whatever life throws at you and other days you just get angry at the smallest upset.
Dogs are just the same, if out on their walk they meet a strange man in a hoodie that is a bit scary and then a loud car goes by which startles them and a dog on the other side of the street starts barking at them and you walk passed a child who is crying and a cyclist goes passed, wow that was exciting your dog would have loved to have chased that! By now your dog is getting quite stressed as things are happening around them that they cannot control or get away from or to, as they are on lead. None of those things have actually hurt or even been that threatening to your dog but when that little old lady with her small dog comes around the corner without warning, it could be the tipping point that just is one stress too much and your dog starts barking at them.
It not actually the fact that your dog doesn't like little old ladies or small dogs, it's the fact that you hadn't realised your dog had been stacking up all these little stressful incidents until they could not cope and the reactive behaviour was their only release. This is called trigger stacking it is where little stressful events build up and if not defused can blow up into an over reaction.
So what can we do to help?
There are lots of fun things dogs enjoying doing that can be beneficial in lowering anxiety or stimulation.
Sniffing is a great way of defusing as it gives them a way of putting all their focus into a task that has many benefits for them, such as food and information. So just by dropping a few treats around you, especially in the grass is a way of balancing out that trigger.
If your dog enjoys a cuddle, stroking ears or belly rub then use that after a trigger event to help soak up some of that stimulation, do it slowly and calmly to bring those levels down.
Playing tug on a toy can get them to forget that dog that just barked at them and in fact will make that situation that just happened a positive event because something fun happen because the dog barked.
Do some speed sits and downs with instant releases as soon as they offer the position (think musical statues) and have fun.
Even if you don't notice your dog stacking those stresses it's worth just making sure your dog gets plenty of opportunities to de-stress during the day in order that they do not get to threshold.
Now be aware each dog has a different point at where there threshold line is as some dogs are very laid back and it can take a lot of events before they hit that line but a dog that is already quite timid or lacking in confidence may have a very low threshold and it may only take a couple of small incidence for them to hit that threshold.