Issues with Huawei stem from its perceived ties to the Chinese government. Its founder was a senior engineer with the Chinese Liberation Army. The concern is that allowing a company that might be beholden to a foreign government access to the US telecommunications network is a bad idea. So far, the Congress has proposed blocking Huawei from any government contracts and advised carriers, ISPs and private citizens against buying the company's products.
The scope of this new probe is unclear, nor are we aware of specific allegations being made against Huawei. However, it signals that the US government is interested in the company's actions at a broader level. If the Department of Justice finds that Huawei violated sanctions against Iran, the company and its officials could face criminal penalties.
Update: We've received the following statement from Huawei: "Huawei complies with all applicable laws and regulations where it operates, including the applicable export control and sanction laws and regulations of the UN, US and EU."