Guangdong - and the Pearl Delta in general - was the heart of the export industry. It made its money mainly through manufacturing and trade, especially with Hong Kong next door as the city of choice for foreign businesses' Chinese operations. Now that China is relying less on trade, out sourcing more of its manufacturing, and foreign businesses are being replaced by domestic competitors, it isn't particularly surprising that the provinces with the better universities and R&D ecosystems are out competing Guangdong.
Mind you, Shenzhen is doing just fine, but Guangdong as a whole is reaching the end of its cycle. It'll need to invent itself again in the new Chinese economy.