Green’s Dictionary of Slang

(UK Und.) a bad time for highway robbery since heavy traffic is likely to impede a safe getaway.

1725		New Canting Dict. n.p.: catching-harvest [...] In a Canting Sense, a precarious Time for Robbery; when many People are out upon the Road, by means of any adjacent Fair, Horse-race, &c.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Catching harvest, A dangerous time for a robbery, when many persons are on the road, on account of a horse-race, fair, or some other public meeting.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.