This one is a pattern that I observed systematically.
Every 2 or 3 months I do resubmit my site map on google.
This seems to lead to a traffic drop of 20 to 25% in traffic to the site for a few days - maybe up to a week.
Would this be some sort of sandbox effect?
I observed the same type of traffic drop when changing or adding ads or campaigns on google adwords.
Now the pattern is systematic enough to be real.
In the last week, I resubmitted site maps multiple times and deleted old ones as well because I cleared the site the previous week.
I removed old pages and outdated material as well.
The results is a clear drop over a few days again.
Because I have been "playing" with these sites maps more, the drop seems to last slightly longer than usual.
This whole thing is only a theory.