The term "bubble" gets thrown around a lot, sometimes incorrectly. It doesn't just mean that money is flowing into an asset class.
Let's take an S&P 500 index fund as an example. Vanguard or whoever does some marketing, and people decide that index funds are a good way to invest, and money flows into index funds. That's not a bubble.
The economy starts doing well, and stocks go up, and a bunch more people decide that they want to be in the stock market, and so they put their money into an S&P 500 index fund, not because they really want to be in the stock market, but because it's what's going up. That's still not a bubble.
Vanguard gets a bunch of money for its S&P 500 index fund, which it has to use to buy stock in the companies that compose the S&P 500. As a result of all the new money coming in, the stock in the S&P 500 companies go up - more than the fundamentals of their business indicate, more than stock in, say, the Russell 2000 goes up. Because the S&P 500 has gone up more, more money pours in to S&P 500 index funds. Now the S&P 500 is going up because of all the money being invested in it, and all the money is being invested in it because it's going up. That's a bubble - a positive feedback loop that has become detached from the fundamentals of the assets involved.
That's a bubble, but it doesn't really get bad until borrowed money enters the picture. If people are borrowing money to invest in S&P 500 index funds, because the funds are going up faster than the interest cost on the borrowed money, now it's a bubble that can cause serious damage when it pops, because it may damage the lender as well as the borrower.
So, for example, people were talking about bonds being in a bubble because bond prices were so high (extremely high by historical standards). That was a flight to safety, not a bubble. People were not buying bonds because they expected bond prices to keep rising, they were buying bonds because they expected other prices to keep falling.