Why is the U.S. wary of the Belt and Road initiative? As you may know, the U.S. has been trying to convince its allies in Europe and in other countries to be wary of the BRI project insinuating that the BRI is a scheme to put countries into debt serfdom. Is the BRI a threat to U.S. geopolitical dominance? If so, how so?
Because USA's position as global superpower is being threatened.
In 1948 USA launched Marshall Plan, economic assistance initiative to help rebuild Western Europe. While propaganda machine loves to claim otherwise, it was not out of goodness of the heart. With discontent by previous national governments and bad post-war living conditions, revolutionary sentiment was at an all time high. Being unable to employ same measures as those used in South America (coups; after break, they are back at it, compare contemporary Venezuela) USA intended to prevent Communist revolution by pre-emptively helping raise standards of living. This also gave USA leverage over assisted nations, influenced their economic systems in way USA wanted and gave USA leadership role over so called "western world" in the upcoming Cold War. Considering that up to that point Western Europe was most influential on the globe (compare: Colonial Empires), it gave USA indirect influence over still-colonised regions.
China is basically trying to achieve same goal. Strengthen economic and political relations while subverting ties to the enemy. Infrastructure investments are a non-hostile and non-adversary way of subverting US influence.
Increased connectivity with China, will obviously strengthen economic ties to China increasing Chinese influence. In face of presence of and global influence of Chinese companies (recent Huawei spat) and fact that seemingly everyone and their mother manufactures everything in China, strong relation with USA will be less and less desirable: can't buy cheap stuff from USA because USA doesn't make anything cheap and USA'a advantage in hi-tech department is decreasing.
Furthermore, land transport across Eurasia or to Africa (like railways, for example) can not be blockaded by navy, and thus is immune to US' main tool for force projection: US Navy. Recent disputes over South China sea, including Chinese construction of artificial islands suggests that decreasing reliance on sea transport might be viable strategy to decrease USA's ability to disrupt trade, more so in unlikely scenario of open aggression.