Mortgage rates are based on bonds and bonds can do funny things on the first and last trading days of any given month. One of the most common "funny things" involves a decent amount of market movement for no apparent reason. In truth, there are always reasons, but on the first/last trading day of the month, they aren't the normal reasons (such as a reaction to economic data), and they're too esoteric to merit an explanation for the amount of movement seen today.

There are some less esoteric motivations in play as well. US bonds often take cues from European bonds when there isn't anything more compelling to offer directional guidance, and European bonds had a bad morning. The spillover to US bonds was apparent in the overnight hours. 

Lastly, some news stories speculated that legal challenges to Trump tariffs could be contributing to higher rates. This highly unlikely given the timing of the news (it happened just after markets closed on Friday). If this were a motivation, it would have been apparent right when trading began for the new week on Sunday night in Asia. Instead, it wasn't until European markets opened that the bond market began pointing toward higher rates.

Motivations aside, rates didn't move too much higher. In terms of top tier 30yr fixed rates, the total damage was 0.03%, thus leaving the average lender right in line with the lowest levels since October 2024.