I theorize that there are two factors: First glaciers are efficient at gnawing at mountains, and second rock is brittle therefore mountains tend to set in the timespans of millenia even if there are no landslides. The second one might be the one that is unpredictable.
In Switzerland there's a tiny town, Brinzauls, which is setting as a whole together with the slope it sits on about three or six feet a year, because in the depth of a few hundred feet there's water flowing through the cracks of the bedrock. There are plans to bore a tunnel along the water-carrying stratum to drain it to stop the movement downwards.
This is just an example of one of the many things that might happen to crop down mountains.
The setting movement is unpredictable because it also depends on the climate. In oceanic climates water could quite literally be dissolving mountains from below.
In Switzerland there's a tiny town, Brinzauls, which is setting as a whole together with the slope it sits on about three or six feet a year, because in the depth of a few hundred feet there's water flowing through the cracks of the bedrock. There are plans to bore a tunnel along the water-carrying stratum to drain it to stop the movement downwards.
This is just an example of one of the many things that might happen to crop down mountains.
The setting movement is unpredictable because it also depends on the climate. In oceanic climates water could quite literally be dissolving mountains from below.