Why is the lanthanide element Thulium malleable?
All the lanthanides are soft metals; there is nothing special about thulium in that regard, AFAIK. For that matter, so are the alkali metals, and gold can be scratched with a fingernail.

Most pure metallic elements are malleable. "This is because of the ability of the atoms to roll over each other into new positions without breaking the metallic bond." Brittleness (the opposite of malleability and ductility) often occurs because of impurities such as carbon, or a metal alloy which separates on cooling. These impurities "pin" dislocations in the metal, preventing them from moving smoothly. It makes alloys harder, as well as more brittle. The early technologists discovered that the alloy bronze was harder than pure copper.