Commentary for Avodah Zarah 97:9

Eliezer refers to a situation in which bread was baked in an oven heated up with wood from an asherah tree. This is another case of “this and that cause” for the prohibited wood and the permitted oven were used to cook the bread. Thus R. Eliezer is the strict opinion—in cases where there are two causes, one prohibited and one permitted, the rule is stringent.