10% comes from the ancient Roman tradition of Decimation https://en.wikipedia.org/wiki/Decimation_(punishment) >> The dual purpose intended was to stiffen discipline amongst the army at large and to demoralise the enemy
I'm sure every executive is calling back to their days studying roman history when they decide how much of a layoff will balance the books/turn the company profitable for its stakeholders.
Though one notable difference is that with layoffs it's an excuse to drop so-called poor performers, but I believe decimation was more or less random i.e. drawing straws.