This effect has been attributed variously to the following factors:
1: The ubiquity of such material on the intarwebz, especially in bulletin board posts that devolve into chan sessions.
2: The fact that deep down you really wanted to search for "pre-teen anime panty shots", but didn't have the honesty to do so. Google knows better. Don't try to fool Google.
The Ecchi Effect can also be regarded as a consequence of Rule 34.
Person 2: "It's the Ecchi Effect, man. You asked for it anyway, you've got safe-search off, don't try to pretend you don't want to 'accidentally' look at naked cartoons."