It seems we missed part of page views in June/July. (data stream error?)
Sharp drop starts around June 16 and (only partial) recovery happens two weeks later.
a 15% MoM drop in page views for all wikipedias combined is extreme 
A peak in some very general subject can always happen (e.g. due to related media coverage).
A sharp drop is much more unlikely, if on many articles, and for an extended period (more than one or two days), but too short to be seasonal.
Both stats.grok.se and vitribyte.com show that same extended drop: e.g. https://www.vitribyte.com/insight/topictrends?p=London_Bridge&w=3m
stats.grok.se also shows full day of missing data near the start of the drop.
And in weeks that follow old level is never reached again.
 I'll try to attach screenshots