It might just be because they have to die somehow. Probably 1 in 4 humans' hearts eventually stop working too.

> The dunk-and-spin maneuver is risky, Yarger notes, as roughly one in every four dragonflies eventually drowns.

It's not that risky, with about a 99.99% success rate. But in their lifespan of a few weeks, they will still be doing thousands of them...

Selection is a balance. It is hard to judge what is being selected for when you only look at one factor (drowning).

Aging versus cancer is an example of balance: one theory is that age related diseases are a side effect of selection forces against cancer: programmed cell death after X reproductions (telomeres) are a general anti-cancer defence but the cell-death has aging effects. Also beware that selection is for successful reproduction: death after reproduction is not so relevant to evolution.