Your dog may have multiple triggers that make him uncomfortable. If you have a dog who reacts to stimuli when outside, it is worth your while to mark down all the things your dog reacts to - whether mild or more strongly. Be aware of what occurs in the course of your day with your dog. Has he encountered any of his triggers? When stressed, the body releases cortisol (a stress hormone) which can remain in the body for 2-6 days.  Even though he may have encountered a trigger on Monday, the physical changes in the body remain for a few days. Your dog's threshold is now slightly lowered so experiencing another trigger or two the following day will likely cause him to react quicker.

My dog doesn't like thunder, children or adults reaching out to touch him or the sound of trucks as they pass. Each on their own, is tolerable. 
But - last night there was a thunder storm. If we went on a walk in the morning and encountered a few people who wanted to pat him and then a delivery truck passed by, he would react in a much stronger manner. He would have started the day with an elevated cortisol level due to the storm. Add to that people wanting to pat him and trucks making deliveries - tolerable just became intolerable.
When multiple triggers "stack", the dog's threshold for tolerance is lowered. That may be why sometimes you are surprised at how your dog reacts in a particular situation. Triggers may have been building up without you realizing it. It's important to be aware of your dog's triggers and how they affect him.