I can’t give you an answer as to why the stock has moved the way it’s moved in the short term of this past week.
What I can tell you concretely though was that Pinterest made the willful decision to lower its initial pricing of its IPO to below its valuation (75 million shares priced between $15-17 for a total valuation of ~$11 billion compared to its original supposed pre-IPO valuation of ~$12 billion). This shows a clear lack of confidence in the company. This is a fact, not an opinion.