When our 15-month-old child goes to sleep without playing enough or when she sleeps after crying, she cries in the middle of the night.
I don't know if the mentioned cause are probable to be exact or there is some other causes for this behaviour?
They may wake up and feel lonely or scared of being alone. When our toddler wakes in the night and starts crying he generally seems to be looking for us, he is 2 and a half and is no longer in a crib.
When this happens we have two options, do nothing and he'll fully wake up and come looking for us, or go to him and gently sooth him with some rubs on the back or head and tuck him back in, reassuring him we are close by, and he goes right back to sleep.
People in general (kids and adults) tend to partially wake up several times during the night, and that might be worse if they haven't gotten lots of activity to tire them out.
I don't know if the mentioned cause are probable to be exact or there is some other causes for this behavior?
My son went through the same thing. The doctor said it could be due to the fact that his eye sight was developing and when he woke up he wanted to see things.
I don't know if this was true or not, but what I did to get him back on track with his sleeping pattern was to walk him around the house, holding him against my chest facing forward. This way he could see, and get the stimulation he required. YMMV, but this worked at that age for me.
NOTE: I am making the assumption here that you have allowed you child to cry a bit, giving them the time to adjust themselves and potentially fall back asleep. The worst thing you can do is go running to your child the moment they start crying. ( as long as its the safe crying, and not the Oh beep type of crying )