One possible reason is a lack of proper nutrition or dehydration. If a mom is not eating well or drinking enough water, especially when she is very busy taking care of her family, her body can become weak and she might pass out. Additionally, certain medications she is taking might have side effects that lead to unconsciousness. It could also be related to an underlying medical condition that she was not aware of, such as a heart problem or a neurological disorder.
In a story, an unconscious mom could be the result of a psychological shock. For instance, if she received some extremely bad news all of a sudden, like the loss of a loved one. Or perhaps she was in a situation where she was overexposed to heat or cold for a long time without proper protection. Poisoning is another factor to consider, either from eating something bad or being exposed to harmful chemicals in the environment.