Without further knowledge, since I wasn't there, I can't speak definitively, but the according to the mother she tried to calm the child. Like I said earlier, very young children obviously do not have a functioning vocabulary to express their wants or needs and crying is the way they communicate that. A 20 month old likely knows a few words, but in the frustration of not being able to accurately communicate using those words, she'll resort to crying again. This is how small children work. It's a fact of life that no amount of parenting changes.
Again, the bus driver needs to grow some thicker skin, or find a new job. On the first page, I posed a question that people were apparently afraid to tackle. If you defend the driver's actions, where do you draw the line?
"Maam, your crying baby is distracting, please leave the bus."
"Maam, your shirt is distracting, please leave the bus."
"Sir, the color of your skin is distracting, please leave the bus."