A source tells ET that Chmerkovskiy actually refused to rehearse with Lachey this past week due to the two not getting along. According to our source, both Lachey and Chmerkovskiy are "stubborn, strong-willed people," and it was obvious from the beginning that they were going to "butt heads." The source also claims the pair has been faking their seemingly close relationship.
The source says their issues "erupted" this past week, resulting in Chmerkovskiy's refusal to rehearse with Lachey in preparation for Monday's episode.
"It was nothing specific, just little disagreements that escalated," the source says.
However, the source notes that Chmerkovskiy will be back next week.