Answer
Unstable or rapidly changing environments in which the odds of survival of offspring and parents are low favor semelparous reproduction.
Work Step by Step
In unstable environments, the odds of a parent surviving their conditions for breeding are low. The odds of their offspring surviving are also low. These species have one-time, large scale breeding events where extremely high numbers of offspring are produced in order the maximize their chance of survival. In more stable environments where both offspring and parents can be expected to survive for multiple reproduction cycles, their lifespans typically allow for this.