We first heard the news from MacRumors.
Simply put, each device faces an issue in production. The iPhone 5S's supposed fingerprint sensor supposedly requires a special color coating to prevent interference. The reduced-price iPhone's extra-thin plastic shell will likely lead to issues surrounding color coating. And the hypothetical Retina display for the iPad Mini is expected to slow down the timing on the device's release.
The original target for the new iPhones was June or July, but the report does not provide a new target.