Some alga attempt to procreate using sexual means. Caulerpa is a great example. There are male Caulerpa and female caulerpa. When Caulerpa isn't doing real good for whatever reason it sporulates. This involves converting some or all of its mass into gametes.
When these gametes are released they contain nearly everything the caulerpa had sequestered up to that point. So, a month or more worth of nutrients & whatever else the caulerpa had sequestered is released into the water column over a matter of hours. This can be a huge amount of phosphate & nitrate. In addition these gametes are free swimming and consume massive amounts of oxygen and produce Carbon dioxide & ammonia. These Gametes are rarely successful at actually becomming fertilized because normally only one clump of algae will sporulate and few of us keep both male & felame plants of the same species. As a result the gametes die and this causes the second wave of problems.
If the sporulation is massive this can actually crash a tank although it's not common.
This is one reason I recommend multiple forms of filtration, live rock, live sand, and a refugium containing algae. If one fails it's much less likely to be catastrophic.