How can the noticeable decrease of backlink numbers in the SISTRIX Toolbox for many domains on September 19, 2012 be explained?

If you analyse any domain in the SISTRIX Toolbox and look at the “link history”, you may see a noticeable decrease in the graph on the 19th of September, 2012. (This change is also visible on the history of linking domains & hosts as well as IPs & networks).

A screenshot of the backlink history for the domain zalando.de around the time of the change.

A screenshot of the backlink history for the domain zalando.de around the time of the change.

We increased the speed at which links that are not found anymore are removed

We often heard the feedback from our users that they wanted “fresher” links. We got to work on this wish and will now remove links that are not found anymore at a much quicker rate. We used to wait several months to see if a link might show up again. Now, this process is shortened significantly. In a few cases this affects the total number of found links, but on average, we do not see any significant movements in the evaluation. The exact date for this change is September 19, 2012.