In layman's terms, why does the market struggle when 10-year bond yields rise? The DOW dropped 177 points today and this was apparently the key reason.
It's probably not worth paying any attention generally to people that give "key reasons" for daily fluctuations in the market. There have been plenty of days in the past where yields have risen and the market was up sharply as well.
That being said, rising treasury rates generally (but not necessarily) makes borrowing more expensive for businesses as well. Higher costs of borrowing can mean businesses are less likely to borrow to invest further in their operations and future profits could be lower.
You could also easily argue that if markets are doing well that treasury interest rates should rise as investors would rather lend money to businesses rather than the government. It's quite interconnected and complicated which is why most explanations for daily market movements are broadly useless.