Looks like President Trump doesn't want Chinese to put hands on US technology. Seems reasonable - QCom is chip producer, probably leading one in 5G technology. Strongest competitor? Huawei, which has connections with Broadcom and Chinese Gov. Now, imagine that Broadcom buys QCom. And postpones some investitions, ofc to save money... Chinese takovers leading position in 5G, pushing US (including military agencies) to use their chips. Domination.
Not to mention that QCom does a lot for military, thus way all that intelligence would be transferred to Chinese.
Threat for US is real, just remember what China did with Cisco routers:
Selling top technologies to worst enemy, that's what this deal would mean. Ofc US rejected.
My poor puts :(