Why are lesbians 91% more likely to be left-handed?

We don't know, and to a degree the jury is still out on how robust that pattern is (there have been some conflicting reports, and purported links between handedness and homosexuality also seem to be different in me and women).

If it turns out to be a robust, general pattern across human populations then, my arm waving (i.e. non-specific) hypothesis would be that this is not a causal relationship but rather that sexual orientation and handedness are both influenced by a common factor during development, probably early in life (e.g. perhaps in utero).

see
https://en.wikipedia.org/wiki/Handednes … rientation

the evidence isn't great at all and most likley I suspect are statistical "ouliers" that are often seen with small underpowered studies.