The 2018 options vested over 24 months. That 24 months is up, so now fully vested. The form 4 for those even says for 2 years of service. True for all four execs.
The 2020 options were from earlier year this. You will only see the vested portion, that is why the numbers are so small.
The only "milestone" is two years of service for the '18s. Nothing at all special about the timing.