Scanning the Wikipedia article on the 737 Max...........
https://en.wikipedia.org/wiki/Boeing_737_MAXSeems to have been / is a lot of pressure to produce as many planes as possible.
"On August 13, 2015, the first 737 MAX fuselage completed assembly at Spirit Aerosystems in Wichita, Kansas, for a test aircraft that would eventually be delivered to launch customer Southwest Airlines.[26] On December 8, 2015, the first 737 MAX—a MAX 8 named "Spirit of Renton"—was rolled out at the Boeing Renton Factory.[27][28]
Because GKN could not produce the titanium honeycomb inner walls for the thrust reversers quickly enough, Boeing switched to a composite part produced by Spirit to deliver 47 MAXs per month in 2017. Spirit supplies 70 percent of the 737 airframe, including the fuselage, thrust reverser, engine pylons, nacelles, and wing leading edges.[29]
A new spar-assembly line with robotic drilling machines should increase throughput by 33 percent. The Electroimpact automated panel assembly line sped up the wing lower-skin assembly by 35 percent.[30] Boeing plans to increase its 737 MAX monthly production rate from 42 planes in 2017 to 57 planes by 2019.[31]
The rate increase strains the production and by August 2018, over 40 unfinished jets were parked in Renton, awaiting parts or engine installation, as CFM engines and Spirit fuselages were delivered late.[32] After parked airplanes peaked at 53 at the beginning of September, Boeing reduced this by nine the following month, as deliveries rose to 61 from 29 in July and 48 in August.[33] "
Interesting if it turns out to be related to manufacturing.