In a nutshell, generational GCs works by scanning live data. Thus, anything not scanned is garbage. While the application is running, it also allocates new data.

If you allocate an object somewhere --after-- that area has been scanned, the GC will treat that area as garbage, and then you will have memory corruption, leading to segfaults and other nice things.

While parallel mark/sweep is certainly doable (Java does this) it's not easy to get right.