There is a feeling among some that anything from outside “frumkite” will damage the yiddishe neshama. Most people stop at language or dress thats which Lvush or Yiddish is so important to them. However some people take it to another level and add the chumra of not eating “Goyishe Foods” and Goysishe foods doesnt mean treif, it means foods that come from another source beside the Alte Heim. They can be made perfectly kosher, they are just not from the Alte Heim.
Sushi has become the symbol of these foods, Its very popular in the jewish community and some have this idea that if you eat sushi it will lead to going OTD because you have now had a “ta’am” for the goyish world,