Why has a company's rating dropped without any associated event?

Over time, financial statements become less reliable as a predictor of future performance. 

The longer it has been since a company filed its last set of accounts, the less weight those figures carry in our models and the higher the perceived risk. As this risk increases, a company’s rating may be adjusted downwards even if there is no visible trigger event such as new filings, CCJs or insolvency notices.

In most cases where a rating falls without an obvious cause, the age and declining predictive value of the last filed accounts is the primary reason.