SLF here who worked for the UK government for too long. Please don't shoot me, it kept me off the streets.
It seems possible that when the FAA allowed Boeing to self certify they assumed a level of Trust. Speculation, but they *may* now feel that Boeing did not act entirely in line with that level of trust. In which case I wouldn't be surprised if the FAA (et al) are now reviewing every document, email and report they received from Boeing ref the newer 737 offerings with a fine tooth comb, in the light of recent experience. Depending on the time scales, they may also feel that now is good time for a detailed review of the stuff they were sent on the Drainliner and perhaps the also 777X. Could take a while.