Hey guys! Few months ago my 2016 F55 started making clunking noise. After inspection by the dealer, the tech removed struts, tightened the center nut on the rear struts. Clunk went away. I think one nut on the right rear was loose(or both, not sure)

1) I am concerned as to why the nut got loose in the first place?
2) Will it be enough to keep it tight? - warranty expires this year, so I want it fixed for sure.
3) Is it a defect in any of the parts and something needs replacing?
4) I thought if factory didn't tighten the nut properly, then why there was no clunk for 3 years and just started happening in the late autumn of 2019?!

Please let me know if anyone had similar situation where that nut came loose and is there a reason for concern?