It's not a belief, it's in the data.I hadn’t read about this before. It was very interesting. Apparently they believe that the tree ring proxy data is good back centuries but that it suddenly became not accurate in 1960. Skeptical science automatically suggested that it was a man-made problem but when challenged they said the cause was not known but was it was suspicious. I’m going to try to find more information on this.
They go on to say that it's likely a range of issues causing the divergence:
Various studies have noted the drop in Alaskan tree-growth coincides with warming-induced drought. By combining temperature and rainfall records, growth declines were found to be more common in the warmer, drier locations.
Studies in Japan and Bavaria suggest increasing sulfur dioxide emissions were responsible.
As the divergence is widespread across high northern latitudes, Briffa 1998 suggests there may be a large scale explanation, possibly related to air pollution effects. A later study by Briffa proposed that falling stratospheric ozone concentration is a possible cause of the divergence, since this observed ozone decline has been linked to an increased incidence of ultraviolet (UV-B) radiation at the ground (Briffa 2004).
Connected to this is global dimming (a drop in solar radiation reaching the ground). The average amount of sunlight reaching the ground has declined by around 4 to 6% from 1961 to 1990.
One study suggests that microsite factors are an influence on whether individual trees are vulnerable to drought stress. Eg - the slope where the tree is located, the depth to permafrost and other localised factors (Wilmking 2008). This paper amusingly refers to the divergence problem as the "divergence effect" so as "to not convey any judgement by the wording" (you wouldn't want to offend those overly sensitive Alaskan trees).