You are trying to fit a square plug into a round hole with your theory, as strikes in Norway are always at this time of year and no other time. If the regular strikes in Norway actually are used to prove this theory, it is a major flaw with the underlying research as large strikes in Norway can ONLY happen in May/June and at no other time of the year.
As for weather and holidays your theory certainly did not work this year. The strike happened after the many holidays of May, and the weather in May and June has been generally worse than average this year
I am not sure if you have realized that the way statistical models work does not require elimination of variance for the related theory to be relevant and validated.
I am not trying to fit, nor have I fit, a square plug into a round hole as much as you are trying to imagine it to be so. Something to do with last year? Well, some may never give it a rest.
Even within this time of the year, strikes in Norway too have tended to be realized more around dates that are holidays, a weekend and/or where the weather was better. When the weather is bad or expected to be bad, strikes are less frequent, less extensive and/or of shorter duration; and when the weather is good, expected to be better or better than expected, strikes tend to be more frequent, more extensive or of longer duration, more so around holidays and weekends. [Who wants to generally take domestic time out more in the relatively colder, wetter weather even within a given month of the year?] Same sort of thing goes for weekends and holidays being a correlative factor for more intense labor disruptions. Not even Norway's labor force is an exception. Norway is just another normal country in this regard too. It shouldn't bother anybody that people prefer to have time off when they prefer to have time off, even if it is not as ideal as they may wish it to be and it involves labor contract disputes.