The tech bubble that some Silicon Valley watchers still don't believe in got a good deal more inflated in the second quarter. 

According to new figures from Dow Jones VentureSource, it was the biggest quarter for private-company equity financings since Q3 of 2000. U.S. startups raised a collective $19 billion in 1,034 separate deals. The total number of fundings rose 12 percent from the first quarter of 2015, while the amount of invested capital jumped 15 percent. 

It's a feeding frenzy, and what's driving it is a widespread belief that times this good can't last, says Jeff Grabow, Ernst & Young's Americas venture capital leader. "Right now you're seeing a lot of companies in the pipeline financing up to make sure they have available capital to grow or prepare for a time when capital isn't so available," he says. "Things can't keep always going up and to the right." 

That said, Grabow doesn't think a pullback of venture capital dollars is imminent. "We could continue to see this for several more quarters as long as we have good economic activity," he says. 

With every succeeding quarter, late-stage "mega-rounds" by multibillion-dollar startups like Uber, Airbnb, and Slack have been accounting for an ever-larger slice of the venture capital pie. That disparity widened in the second quarter, with 60 percent of all dollars going into later-stage startups and the top nine deals claiming 23 percent of all capital. 

"I'm surprised the numbers are as strong as they are compared to where they were," says Grabow. "These are big numbers."