Why are filled slopes vulnerable to slope failure?

Filled slopes constructed in many decades ago are mostly sub-standard. The relative density of filled slopes may be below 85% and is readily subjected to liquefaction. To rectify the situation, the sloped are reconstructed by excavation of 3m measured vertically from slope surface. Then, compaction should be carried out in thin layers to achieve in-situ density of not less than 95% of maximum dry density. After compaction, the compacted layer would not vulnerable to liquefaction failure. Moreover, it is less permeable than loose fill upon compaction and prevents water entry into underlying soils inside the slope.

For the case of Hong Kong, most fill slopes constructed before 1977 were formed by end-tipping so that they are in a loose state and poses hazard to developments nearby.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.