It's a lot more unusual to have an h after an r than an h after a t, even in German. So if you vaguely remember there's supposed to be an h somewhere, it intuitively makes more sense after the t.
In German, "th" is uncommon (non-existent?) in words that aren't overtly borrowed from Latin/Greek, so you get overcompensation like "Ethymologie". "Rhythmus" trips me up every time.