Surprisingly enough, one reason could be a dull knife that exposes you to more of the irritant.

The duller the blade, the more it was liable to bend the onion’s skin before cutting. These delays stored elastic energy that built up pressure in the vegetable before ultimately slicing open, resulting in a more explosive release of juice. [...] those droplets fragmented while flying through the air to create an even more diffuse mist of all-natural mace.

Dull knives can be such a nuisance that they even create as much as 40 times as many droplets as a sharper alternative. Meanwhile, faster cutting speeds generated four times as many particles as slower rates.

a sharp knife and a slow approach will most consistently minimize the undesirable effects of onions.