"With the failed 1846 harvest as prologue, many merchant houses bought corn and other foodstuffs forward, expecting higher prices in the future. By the time these contracts matured in mid-1847, prospects for a strong harvest that summer caused spot prices to fall sharply, catching many speculators short." [i]

                             Wheat Prices and Weekly Wheat Imports, 1844-50 [ii]

References:
[i] Narron, James and Morgan, Donald P. Crisis Chronicles: Railway Mania, the Hungry Forties, and the Commercial Crisis of 1847 (Liberty Street Economics
https://libertystreeteconomics.newyorkfed.org/2015/06/crisis-chronicles-railway-mania-the-hungry-forties-and-the-commercial-crisis-of-1847.html , accessed 05Nov20)
[ii] Campbell, Gareth, "Two Bubbles and a Crisis: Britain in the 1840s" http://www.cliometrics.org/conferences/ASSA/Jan_11/Campbell.pdf.