Melkamu Dugassa Erena Auxin-producing bacteria in vermicompost can stimulate primary roots development and it will delay plant emergence. You can measure root length in control and in pots treated with vermicompost
We cannot know 'why'. But we can say that germination is slower in vermicompost. It is hard to isolate the specific chemical that might be the culprit, but you could use a proxy, such as root development versus shoot growth to determine if it might be hormonal, as Alex Ignatov stated :)