Plants die from a variety of ways, and if you aren't regularly maintaining your plants and checking up on them to make sure they are fine, they will die. Also, if you don't know some of the main reasons why plants die, you won't know what to do to prevent your plants from dying. Here are some reasons to answer the question, "How do plants die?"
Without enough water, a plant becomes dehydrated and plant cannot survive, therefore it dies. This can be caused by lack of watering, drought, or even too much sun which dries up the soil so the plant doesn't receive enough water. Dehydration can also happen even if there is regular rainfall and regular watering by you because of poor drainage, which can prevent water from reaching plants' roots.
When disease strikes, it starts breaking down the plant, and then kills it. There are many potential diseases that can affect your plant, many of which can end up being the reason why your plants die.
Malnutrition can happen if there aren't enough nutrients in the soil and you neglect to add fertilizer to your plant. This can also happen from overcrowding and competition too since too many plants in one space will compete for the same nutrients which the soil may not have enough of to feed all the plants. Without enough nutrition, your plants will get sick first, and then slowly die because they will need certain nutrients to survive.
Too much water stored near the roots from improper drainage can drown your plant. Also, too many nutrients can be bad for your plant, which will cause it to die since plants need to be fed in moderation.
As living organisms, most plants cannot handle extreme heat which can dry them out and scorch their bodies enough that they die and in extreme cold, they can freeze to death.