He won the AL batting title that year with a .361 AVG. Yet he never hit .300 or higher again and his lifetime avg was just .271 (all data is from Baseball Reference and Stathead).
His OBP that year was .487. His career OBP was .374 and his next best was .402 (in 1960 in only 428 plate appearances).
His SLG was .662. His next best was .531 and it was .488 for his career.
His OPS+ was 201 that year and his next highest was 149. Lifetime it was 139.
I wondered if his flukiness was balanced against both lefties and righties.
This table shows his OPS vs. righties relative to lefties for each of his 14 full or close to full seasons
1960 1.50
1961 1.57
1962 1.43
1963 1.36
1964 1.54
1965 1.15
1966 0.94
1967 1.31
1968 0.97
1969 1.56
1970 1.21
1971 1.27
1972 2.11
1973 2.39
The 1.57 in 1961 is the highest until late in his career when his performance against lefties went down quite a bit. But it was just a bit higher than 1960, 1964 and 1969. So this does not indicate a great imbalance.
But, I also calculated his OPS vs. righties relative to the league average of all left-handed batters vs. righites (and the same was also done for vs. lefties).
Here is his year-by-year OPS vs. righties relative to the league average of all left-handed batters vs. righties:
1960 1.21
1961 1.62
1962 1.25
1963 1.26
1964 1.19
1965 1.24
1966 1.15
1967 1.22
1968 1.20
1969 1.24
1970 1.13
1971 1.30
1972 1.24
1973 1.18
The ratio in 1961 is by far the highest at 1.62 with the next best being 1.30. So a great year for him vs. righties.
Now for his year-by-year OPS vs. lefties relative to the league average of all left-handed batters vs. lefties:
1960 1.02
1961 1.21
1962 1.04
1963 1.03
1964 0.94
1965 1.30
1966 1.39
1967 1.08
1968 1.44
1969 0.92
1970 1.16
1971 1.23
1972 0.65
1973 0.55
He had 1.21 in 1961, but that is only his 4th highest ratio. He had four that were higher: 1.44, 1.39, 1.30 and 1.23.
So he had, compared to the rest of his career, a fantastic season against righties. But against lefties, it was just good.
Update March 25: From 1960-73, Cash had an OPS of .918 vs. righties while all left-handed batters had .733. His ratio is 1.25 (.918/.733). So his 1.62 ratio in 1961 was far above this.
Over the same period, his OPS vs. lefties was .696 while all all left-handed batters had .625. This ratio is 1.11 (.696/.625). His 1.21 ratio from 1961 was only slightly above this.
I only looked at 1960-73 since he did not get many PAs in 1958, 1959 & 1973.
There can be some idiosyncratic things going on here. For example, 20 of his 162 PAs vs. lefties in 1961 were against Whitey Ford. He had just a .417 OPS vs. him that year.
Cash only had 52 career PAs against Ford. So it is possible that 1961 was an unusually tough year for him in terms of the quality of the lefties he faced. But to conclude that would require looking each of his seasons to see who he faced. Also, in 1961, Ford seems to be the only good lefty that he faced fairly often.