On Tuesday, Wizz Air's biggest investor, American equity firm Indigo Partners, announced that it had sold most of its holding in the low-cost carrier. The share placing, done by Barclays Bank, involves 7.69 million shares and is due to be completed by Thursday. This sale brought the company's stake in the airline down to just below 9%.

Shares of Wizz Air dropped by 6.5% today as Indigo Partners announced it had sold $555.5 million worth of stock. The reason for the U.S.-based private equity firm selling its shares is unclear. The airline will not receive any proceeds from the sale. 

Wizz Air, listed on the London Stock Exchange in 2015, has seen an upward trend in valuation as the airline has seen significant growth, with a fleet of 120 Airbus A320-family aircraft serving 150 destinations across Europe. As of late, the Hungarian-based low-cost carrier has been an aviation success story. 

One of the most likely reasons for the share sale was to ensure that Wizz Air complies with EU shareholder regulations following the United Kingdom's departure from the EU. It stipulates that airlines wishing to operate in the EU block of countries must have ownership of over 50% of shareholders within the EU. It appears that demand remains firm for shares in European low-cost airlines.