Hi Georg,
I think the behavior you see can be explained by an overloaded download server. From the initial downloads graph you can see that there are on average 80.000 downloads a day. From the update pings and update requests graphs you can estimate that there are about 800.000 active Tor browser users. So, when there is a new version of Tor browser the number of update requests massively overloads the download server. The saw-tooth form of the update requests graph is what you expect in this situation. First you get an update request from all users, the next day you get a request from all users minus the users who were updated the previous day (max 80.0000) and so on.
I wonder if it is possible that failed downloads are counted too? That would explain the spikes. But systems that are so heavily overloaded can generate all kinds of weird results.
One thing bothers me. The update requests graph never touches zero. It should, because that would mean that all Tor browsers have been updated. 100.000 seems to be the lowest value.