Hello,

The reason behind this more reactiveness of Lanthanides than Transition elements is one of the consequence of Lanthanoid Contraction.

The higher reactivity is due to the larger size of lanthanide atoms, because of Lanthanoid Contraction. The valence (outer shell) electrons of these larger atoms are farther from the nucleus, less strongly attracted to the positive charge of the nucleus as a result, and thus are removed more readily.

I Hope you find the answer helpful.

THank you.