According to the US Treasury, apparently this is due to the fact that Broadcom has connections with foreign parties, thus the potential national security risk. However they did not reveal who those parties might be. As it stands, the Treasury Committee on Foreign Investment has issued an order to Qualcomm to delay a shareholders meeting for 30 days.
It also appears that their concern might have something to do with Huawei, where if Broadcom were to buy Qualcomm, Huawei could indirectly gain dominance in 5G cellular development. This is not the first time that the US government has had their doubt of Huawei, where previously it was reported that US intelligence agencies warned against buying Huawei and ZTE phones.
That being said, it is unclear as to how this will play out and whether or not despite the US Treasury’s misgivings, Broadcom will proceed with their takeover plans anyway, so we guess we’ll just have to wait and see.