A quick Google search indicates that there are detergents that call themselves natural that have ingredients that can come from non kosher sources and there is nothing about these that would seem they are not edible for a dog. They do say they are non-toxic. I am not sure what the halacha would be if a table cloth or towel was washed with a detergent made with real tarfus or chelev and hot food fell on the table cloth or was touched by the towel (Neither an uncommon occurrence). Perhaps people would be concerned even if there was no real problem. I guess it depends on how much remains after a rinse.
There are also many sites where people express their interest in making sure their products, including detergents, are free of animal products.
Seems to me a reason why a detergent manufacturer would want a kosher certification, even though from a kashrus perspective there is no reason to need one.