It is hard to remember what it is like to be 2, 3 or 4 years old. And sometimes we forget to put ourselves in their shoes.
They have come so far in the first 2 or 3 years of their lives, and we therefore overset our expectations of them - we expect them to focus, prioritize and remember way more than they are really capable of.
Think about the stress you feel when there is a change in your life. It could be as small as changing your morning routine to changing departments at work or changing jobs. Young children experience the world intensely and therefore even a small change that we would not even consider a change could throw a toddler off - if s/he perceives it as a change in his or her routine.
Also, research has shown that children at that age cannot select and prioritize information from the flood of sensations that reach them. Parents find toddlers will have potty accidents while doing something that they enjoy and are focused on such as watching TV or playing outside - so that they fail to notice and or remember their physical needs.
Children's memories are also a lot more situation specific than we realize. Toddlers are not capable of thinking ahead about their potty training needs. A small change such as slightly different clothing or being away from home can result in an accident because they wait too long before deciding to go and then the change has thrown them off and it is too late.
Even though children are learning to speak at that age, they still cannot always verbalize what they are feeling i.e. it hurts to urinate because their skin maybe irritated or it hurts to poop because they are constipated. Unable to articulate the problem, the toddler ignores the feeling until an accident occurs.
Since your toddler also does not know your expectation around this situation, s/he is actually looking for information to correct the problem and may in fact cause accidents on purpose to see your reaction and hope to find a solution and figure out the rules and expectations for the situation.
So just keep in mind and put yourself in your toddler's shoes - you are asking them to learn a lot and then consistently follow through each time. So, like all the skills such as walking, eating, and talking that you have taught, encouraged and nurtured so far, potty training is also a skill that requires you to teach your toddler to pay attention to his/her physical needs, think ahead, focus and plan his use of the potty or toilet.
