Wordorigins.org

Nowadays, to frog march someone is to pin their arms behind their back and hustle them along with a person at either side. The term is usually used in reference to prisoners in police custody. From J. Ferguson’s 1931 Death Comes To Perigord:

Cæsar slewed him round, and forcing both arms behind his back, got ready to frog-march him to the door.

But why frogs? The answer is that the modern concept of frog-marching is not the original. Originally, frog-marching was carrying a person face downwards, with a man holding each limb. The metaphor comes from the idea that a frog crawls on its belly. From the Evening Standard, 18 April 1871:

They did not give the defendant the "Frog’s March".

This citation from the Birmingham Weekly Post of 15 November 1884 makes the concept clearer:

Deceased was ‘frog’s-marched’–that is, with face downwards–from Deal to Walmer.

(Source: Oxford English Dictionary, 2nd Edition)