On my environmental task today, a "trees expert" responsible for many of the saplings that have been planted by volunteers on Reading verges, suggested that the landslip was caused by trees having been removed from the sides of the embankment. One might guess that the stumps had been treated, resulting in the decay of the roots and subsequent soil disturbance.
The embankment in question was built in 1900, when the line was redoubled. At the time there was a lot of press comment about how unstable both embankments and cuttings were in that area, with slips quite common. Perhaps surprisingly it was cuttings that were worse for that, and in particular just after being constructed - one big slip happened during the 1900 works.
At this specific site there was a big slip in 1951, and on the other side (i.e. in the original embankment) a bigger one in 1960. There is a culverted stream there, and the land is marked as "liable to flooding" on old maps. So after a lot of rain the embankment is sitting in a swamp. One of the 1960 articles says about cuttings that the problem of landslips "has now been largely overcome by the planting of trees".
But I gather that has had adverse effects too, when the trees get too big and the ground is clay. The trees exaggerate the wet/dry cycle by pumping out water is the summer, and this breaks up the clay and allows slip planes to form. No doubt the recommended action plan now starts with "if possible, don't start from here".
