As we get into the new week, Treasury yields are still sticking with the retreat from last week. 10-year yields are resting just below 5% today despite the fact that the inflation equation hasn't changed, as oil prices continue to push higher last week amid lingering tensions in the Middle East. And yet, tech stocks didn't exactly celebrate last week as the Nasdaq posted its first back-to-back weekly decline since July.That sounds a little bid odd, doesn't it? Aren't lower bond yields supposed to be good for tech stocks?Well, normally yes. However, I reckon that the story last week was more about where investors felt comfortable in putting their money. The question now is whether that story is going to keep up as we look to the weeks ahead.Let's first dial it back and look at things more simply.Imagine you have $100,000 to lend. One option is the US government, which is generally considered a relatively safe - if not the safest - borrower. The other choice is a tech firm that is spending billions in building data centres, hoping that those investments will eventually generate enough profits in the future.A few months ago, investors might have been more enthusiastic about the second option. But with questions mounting over the enormous debt financing behind the AI boom, some investors are understandably becoming more cautious today. And we're seeing that reflected in the bond market.10-year Treasury yields have now slipped back towards 5% even with oil prices remaining elevated and inflation concerns still lingering. Part of the story could be that investors are looking for the relative safety offered up by government bonds. And when more people buy Treasuries, their prices rise and yields fall.However, I would argue that the money flow is what traders should be watching out for. While we are seeing more appetite to hold government debt in the past week, we're also seeing much more cautious conditions in terms of lending to companies heavily exposed to the AI boom.But wait, how can we tell that? Well, you can see some of those nerves showing up in credit default swaps (CDS) which essentially measure how much investors are willing to pay for protection against a company failing to repay its debt.For example, let's take a look at CoreWeave. Its five-year CDS surged above 1,000 bps earlier this month. Meanwhile, Oracle has also seen its CDS approach 300 bps. Put simply, investors are demanding much greater compensation for taking on the risk of lending to these companies.Now, 1,000 bps might not sound like much but think about it this way. If you wanted to insure $1 million worth of CoreWeave's debt against default, it would cost you roughly $100,000 a year at that level. That's around 10% of the amount you're trying to protect, every single year.When you put the dollar terms to that number, suddenly it turns out to be an awfully expensive insurance bill. And that tells us just how nervous investors have become about the company's ability to repay its debts.For some context, Oracle's 300 bps would translate to roughly $30,000 a year for the same amount of protection. That's considerably less than CoreWeave, but still a sign of growing unease.And considering how much money these tech companies are spending on AI infrastructure, one can understand why investors are starting to ask some really tough questions. After all, building data centres is one thing but generating enough profits to eventually pay back all that borrowed money is whole other thing.Now, that is not to say the AI boom is suddenly falling apart. Demand for AI infrastructure remains strong and there's still plenty of money flowing in the space. However, it is clear that investors are starting to become more selective about which companies can justify those investments and, perhaps more importantly, which ones can afford to keep spending.So, how does all of this tie back to falling Treasury yields?The argument here is that we could be seeing some early signs of a shift in market sentiment. That is not to say that investors are abandoning risk altogether. However, they may be becoming less comfortable taking on certain types of risk - especially when it involves highly indebted AI companies.And if that continues, it could create a rather dicey situation for tech stocks. Lower Treasury yields would ordinarily offer some support to valuations. But now, that benefit could be outweighed by growing concerns over corporate debt, financing costs and future profitability.To keep things short, there's a big difference between yields falling because investors are optimistic about lower borrowing costs and yields falling because they're getting more worried about where else to put their money. And right now, there's a good chance that we are seeing the latter be part of the picture in broader markets. This article was written by Justin Low at investinglive.com.