Let me give it a shot with actual references, and years of paying attention, not whatever I heard the talking heads say last night.

The US perceives the economic relationship with China to be one way. Prior to the recent trade flair ups, China was given preferential treatment, perhaps as part of a strategy to create a geopolitical ally instead of an enemy through trade. This appeared to be working for several decades with China opening up to the world and giving the appearance of freer markets and more democratization as they became wealthier. As their economy continued to grow and modernize, the trade imbalance between the United States became huge ($539 Billion in imports in 2018, only $120 Billion in exports [0]). This means that each year hundreds of billions of dollars flows directly from the US economy into the Chinese economy. In addition, with President Xi the country appears to be sliding back towards a nationalistic totalitarianism.

While both countries can still benefit from this trade, the US government is taking the stance that China has been benefiting and the US has not. The reasons that are most often cited are that the US was open to almost all Chinese imports, while many US imports were restricted or taxed more heavily than local goods [1]. Worse than that, if a US company wanted to operate in China, they were required to do so through Chinese partners that would own half of a joint venture, and intellectual property would be shared with them. After a short term benefit, all of a sudden the Chinese partner would be running their own factories, and competing against the US company with the US companies own technology. These companies are often state owned and receive funding that is considered to be anti free trade and is generally illegal in most international agreements (see the legal squabbles between Airbus and Boeing for reference). Finally the US company would be squeezed by the Chinese government (national or local), and all sorts of issues that wouldn't impact the local company would mysteriously befall the foreign company. After local Chinese companies were created, funded and protected in China by the Chinese government, they could compete with their former US partners around the world.

Therefore while the US market was open to
China, the Chinese market is not really open to the US. Finally, it has been widely known for a long time that China hosts state sponsored industrial espionage units that hack companies around the world and steal intellectual property to provide to Chinese companies[2]. US companies will spend billions of dollars researching how to do something, and then a Chinese company can just start producing it, sometimes even before the US company does. Finally, it seems like laws are not enforced in favor of US victims. If a US company steals and sells Chinese IP, they can be successfully sued in US courts. If a Chinese company steals and sells US IP, no enforcement is taken. This is a huge problem especially for small inventors, the moment they create something they lose the market to cheap Chinese knockoffs, and have no power to protect their brand or invention.

For these reasons and others, China is viewed as having a predatory trade relationship with the United States. I've heard that Huawei in particular is known to have gotten a big jump by outright stealing Cisco's router and selling it[3], and has continued with similar tactics, alongside the accusations that it is essentially an arm of Chinese intelligence services and provides backdoors in their products. I've actually heard from Chinese friends that Huawei was very proud of their ability to steal Cisco's router and it is used as a case study.

And while all this is happening, the US had a special trade relationship with China such that it was cheaper to ship goods to individuals in the US from China, instead of from inside the US itself[4].

So basically a viewpoint that I tend to agree with is that this is all about resetting the trade relationship to a normal one between peers. The US is no longer supporting a third world country, but dealing with a fellow world power that is fully capable of taking care of itself. I am confused myself why it is being portrayed any other way, and what the motivations are for doing so. It seems to me that the US has demands to be treated fairly and have an equal trading relationship (according to their point of view), and if that doesn't happen the US can impose tariffs that will rebalance trade between the two countries.

[0] https://www.marketwatch.com/story/heres-all-the-stuff-the-us...
[1] https://www.export.gov/article?id=China-Import-Tariffs
[2] (pdf) https://www.fireeye.com/content/dam/fireeye-www/services/pdf...
[3] https://blogs.cisco.com/news/huawei-and-ciscos-source-code-c...
[4] https://www.forbes.com/sites/wadeshepard/2017/11/05/how-the-...