Story

Not being a biologist, I can't respond authoritatively, but I am guessing that this evolved in the same was as many other "mutualisms" (an official term worth looking up if you are genuinely curious). Another famous example of co-evolution is the orchid (Angraecum sesquipedale) which has a 16 inch long nectary. When it was first discovered, Darwin postulated that there must be a species of insect with an insanely long tongue that feeds on (and pollinates) that flower. Without a moth with a 15 inch long tongue, the flower wouldn't survive, and vice versa. That moth was observed in 1992 (way, way after Darwin had died).

So back to mutualism, any creature that visits a particular plant for a number of reasons (food, resting place, etc.) and picks up pollen allows that particular plant a greater opportunity for genetic diversity. If those visits lead to better reproductive success (for both creature and plant), you begin to have a generation-long feedback loop of changes (however small) that led to a mutually dependent relationship.

In this particular instance (and this is hypothetical), let's say that a plant secretes something that the bats find tasty. That isn't unusual. These particular plant exists in a nutrient-scarce environment, so being able to attract the bats (and their feces) leads to greater reproductive success for that particular plant. Minor mutations that occur in every generation may lead to some plants being more tasty, and some plants being less tasty, to the bats. Those that are more tasty get the bats to come by more often. Bats coming by more often leads to healthier plants that reproduce better. None of this is purposeful on the side of the plants. It just works out that some minor genetic changes due to mutation are beneficial, and some are not. As time goes by, both species (plant and animal) that are benefited by the relationship are also shaped by it. Bats that visit these plants to eat or use them as shelter have more babies (meaning more bats that would visit vs. bats that would not visit), and the plants that attract the bats and the bat's feces are more likely to reproduce as well (so more plants that attract bats, and fewer plants that do not attract bats. Pretty soon, especially if this is one of the stronger environmental pressure on their genes over generations, you have two species that are very different than the first plant that attracted that first bat. The taste of the plant is just one of the many minor things that would change, of course. Shape changes over time, and some changes are beneficial (more bats visit, leaving feces, meaning proliferation of that particular shape) and some are not beneficial (fewer bats visit, plants don't reproduce as well, those variations die out or become scarce).

This is a very simple hypothetical of a relationship that developed over many, many generations. This kind of relationship isn't all that rare, or all that surprising (although we are surprised by the funny things nature does, this being one of those particulars).