ややもすれば

Etymology
+ particle +, the  of  + hypothetical conditional particle ; literally "if one does slightly [then the subject in question will...]", i.e. "easily, inclined to".

The superimposition of the kanji onto, a component with a different meaning than the superimposed kanji, is only in this set phrase, and can be understood in the context of movement, as in "if one [ever so slightly as] moves [then the subject in question will...]".

Adverb

 * 1) easily; inclined to