It's not that the cause is not known here ... Quite a lot you can understand from the existing diagnostics (Activity Monitor, vm_stat, smartmontools...).
Specifically, Big Sure 11.0.x-11.2.x exposed irrational swapping behavior right after the reboot, when kernel_task was writing ca. 1TB / day or so even when there were 3-4Gb RAM free and memory pressure level around 25%. The only possibility here is that OS swapped out memory pages to disk when it's not needed, and likely with a 5-10 multiplier, i.e. swapping algorithm didn't work properly.
11.3 introduced more improvements: fixed swap use right after the startup (it's 0 for a much longer time), but the issue resurface after the swap use reach ca. 5Gb., so I was getting few hundreds Gbs written per day.
11.4beta1 works best for me so far. Apple tweaked the algorithm to suppress the swap activity, and average memory pressure is around 50% instead of 25% before, with moderate swapping activity (few tenth Gb/day).
Use case matters in a sense that certain activity (browsing with a lot of tabs) may provoke VM thrashing and random excessive swapping faster than other less memory-intensive workloads. But OS issue is always there, and processes are not culprits.