The launch of Balmain X H&M was insane - all around the globe, people lined up for hours to get into H&M stores to grab hold of the collection from Balmain, a Parisian fashion house. Police were called, shelves were emptied.

But why was the hype so out of hand? Why were consumers crazier for Balmain X H&M than they were for Alexander Wang's designer collection for H&M?

The items for Balmain X H&M were so coveted that the H&M website went down for part of the day.

"We have a lot of visitors on the site right now. This is due to the high level of interest in our latest collection," the error message read.

Consumers were grabbing items and posting them on eBay, where bids were about $1,000 more than their retail prices.

"It seems like so far the response have been overwhelming," Hacan Andersson, an H&M spokesperson told CNBC by phone.

One thing's for sure: the marketing for the launch of this designer collection was impeccable. Millennial trendsetters like Gigi Hadid and Kendall Jenner featured in the advertisements for Balmain X H&M. Ads were in your face all over social media.

"There's always a lot of interest in designer collections. For example, when we hosted Alexander Wang last year, we had great interest from customers, so it's not like this is happening for the first time," Andersson said.

But Balmain's collection seems to have grabbed the attention of consumers like never before.

Tiffany Hogan, an apparel analyst with research group Kantar Retail, told CNBC that there's a good reason for the success of Balmain's collection compared to Alexander Wang's: droves of young millennials, influenced by young trendsetters by a great PR effort.           

"Honestly the PR around it was even bigger than it was with Alexander Wang, which was more understated in PR terms. This time they played up to millennial cohort, really appealing to the fact that Balmain is a younger brand," she said in a phone interview.