What: Shake Shack Inc (NYSE:SHAK) shares were cooking last month as the stock jumped 13%, according to data from S&P Capital IQ. The stock soared toward the end of the month as investors were encouraged by a lack of selling following the expiration of the post-IPO lock-up period.

SHAK Chart

SHAK data by YCharts

So what: As you can see from the chart above, July was a wild ride for the burger chain. In fact, the stock was down nearly 20% at one point before surging toward the end of the month. Shares had been falling steadily from their peak above $90 in May as concerns about the company's valuation mounted.

Source: Shake Shack

After falling below $50 in mid-July, the stock got a bit of a jolt -- at the end of the lock-up period, when underwriters and insiders neglected to take advantage of their first chance to sell stock since the IPO, newfound confidence helped shares spike in just a couple of days. Short-covering also likely contributed to the run-up, as 38% of the company's stock had been sold short as of July 15, creating the high possibility of a short-squeeze.

Now what: Shake Shack shares have been volatile since the company's January IPO as determining a fair valuation for the much-loved burger chain has been difficult. The company is barely profitable overall, but its average unit volume sales above $4 million are easily the best in the fast-food industry, and restaurant-level margins are strong as well, meaning long-term profits should not be a concern.

The biggest question mark for investors remains Shake Shack's unit growth. The company has promised to expand by about 10 restaurants a year for the foreseeable future, and sees room for 450 locations in the U.S. That could support annual contribution profits of $300 million to $400 million or more, but the company may have to do more to convince the market of its ultimate value as it already carries a market cap around $2.5 billion. International licensing arrangements remain a lucrative option, and the chain recently added a new ChickenShack sandwich to its menu that could bring in new customers.

Expect the volatility to continue for this fan-favorite stock as opinions are strong among bulls and bears, and look for a big swing Monday when the company reports second-quarter earnings.