First, the Chinese fishing fleets increasingly are venturing far away from their own shores, just as fleets from other nearby countries are also venturing further from their own shores. This is because shore fishing is drying up, due to unsustainable overfishing and ecological collapse. China's distant fishing fleets are thought to have caused the deaths of impoverished North Koreans, by taking over catches from their waters (https://www.nbcnews.com/specials/china-illegal-fishing-fleet...). They are also taking over West African and Antarctic waters (https://slate.com/news-and-politics/2020/09/beijing-fishing-...). They even have tried to fish off the Galapagos using giant factory fishing ships that allegedly turned off their AIS maritime tracking signals to avoid detection (https://www.theguardian.com/environment/2020/oct/21/china-sq...). China operates the world's largest fishing fleet, with the largest volume of catch, and their scale of operation threatens environments and fishing economies worldwide.
Secondly, the Chinese fishing fleet is highly aggressive and often violent. The NBC article I linked above details an incident where a large number of Chinese fishing vessels took shelter in a South Korean bay to ride out a storm, and also destroyed the environment during their short stay, illegally dumping oil, dragging anchors, and more. The LA times article I linked here also details how bigger Chinese fishing vessels (often with steel hulls subsidized by the Chinese government) will ram smaller fishing vessels from countries like Vietnam or the Philippines. They often steal equipment off those vessels, and even their entire catch, and leave the ship adrift and in need of repairs. And as the LA Times article noted, there are barbaric incidents where crew will hurl hammers or cement blocks at foreign crews of smaller vessels, or even beat them.
Thirdly, encroachment in South China Sea waters is enabled by the increasing aggression of the Chinese naval fleet. Despite naval exercises by the US to assert rights of navigation, China has continued to expand a web of artificial islands that host barely-hidden naval installations with airstrips, missiles, and more. Although the US has made public statements asserting that China's claims to the South China Sea are unlawful (https://www.state.gov/u-s-position-on-maritime-claims-in-the...), that has not changed much on the ground for smaller nations who are victims of China's bullying on the high seas. In fact, in 2020 China's operations in the South China Sea are more aggressive than ever, with COVID serving as a convenient distraction for increased naval imperialism (https://asiatimes.com/2020/04/chinas-covid-19-imperialism-sp...). With increasingly frequent incidents where one country detains another country's fishing vessels and crew, the situation is becoming charged and tense (https://www.cnn.com/2020/10/12/asia/malaysia-china-japan-sea...). China is accordingly stepping up its military activities, with naval exercises (https://www.globalsecurity.org/wmd/library/news/china/2020/c...) and testing of anti-ship ballistic missiles (https://www.thedrive.com/the-war-zone/37662/chinese-long-ran...).
Taking all this together, I can't help but view it as a complete failure of other Asian countries to cooperate against a mutual aggressor, and a failure of the liberal West to manage and contain China to the interest of many nations. Empty diplomatic statements, political posturing, and ineffective performative naval exercises are not the answer to a violent and aggressive actor. But no one is stepping up to answer the calls for help from poorer countries like Vietnam.