This paper analyzes bidding strategies in the German control reserve market, a multi-part pay-as-bid auction. We present a methodology to investigate multivariate time series under consideration of exogenous factors and apply it to real data. As results, we examine exogenous drivers of control reserve power prices, identify the efficient set of bidding strategies and analyze the serial correlation of applied bids in the period 2015 to 2017. The efficient set consists of two clusters - increased capacity price bid with moderate energy price bid and increased energy price bid with moderate capacity price. Combined with game-theoretic rationales, we are able to explain the price development and draw conclusions on how to prevent the application of bidding strategies in the control reserve market.