I'm neither leftwing nor a vegan. I just found the following line of thought to be an interesting logical problem.
There are many animals killed in mechanical crop harvesting. Clearing native vegetation, or even just utilizing mechanical harvesters, kills many sentient beings. There seems to be an active debate over whether or not enough animals are killed to justify the argument that eating cattle harms fewer sentient beings than eating vegetarian, but there is no question that many animals are killed in the cultivation of plants.
One way to avoid this is to utilize more human labor. Humans are less likely to indiscriminantly kill animals. Farming processes that require significant amounts of human labor, such as having humans pick fruit and vegetables, means that fewer animals are going to be killed by mechanical harvesters.
Vegans (and ethical vegetarians) are more likely than the average population to have political beliefs that see low wage occupations performed by immigrants as a form of exploitation.
Until they succeed in getting at least some agricultural companies to raise wages to levels that they deem acceptable, leftwing vegans are forced to choose between exploiting immigrant labor or buying products whose production required the deaths of numerous small mammals.
One way to get around this is to not think about or discuss the problem of small animal deaths caused by food production methods. To the extent that the problem of animals killed in plant cultivation is ignored, it is evidence that being a vegan isn't actually about not harming animals.