It wasn't just the hog or cattle situation alone. Both cattle and hog numbers crested in 1955; the output of meat was enormous. This is one reason. There were also other factors indicating a need for production adjustments.
Thompson, Sam H.
"A Situation Report - "Why the Livestock Price Stump?","
Iowa Farm Science: Vol. 11
, Article 3.
Available at: https://lib.dr.iastate.edu/farmscience/vol11/iss4/3