They don’t specify, but based on the period they’re talking about I’d put money on it being related to the cotton scandal, to pripiski - that is, the Soviet tendency to make up production figures. When glasnost happened in ‘88 the fiction collapsed, although not immediately - most cotton producers continued to bullshit about their numbers until the mid 90s, while the industry dwindled due to lack of water for irrigation and desertification.

The article suggests it's unreasonable numbers in the original Uzbekistan data source and that other datapoints may have been worse, the authors just didn't correctly execute their basic checks.

"It turns out that Uzbekistan wasn’t even the biggest outlier, but that the version they had published had the axes cropped so you couldn’t see the outliers..."