In contrast to rationalist models, people do not always have singular goals or even explain other people's behaviour as goal pursuit. Elsewhere, counterfactual accounts determine ways an event may be modified to measure the effects of different causes. We explore how people explain surprising behaviour in two online experiments, with the help of computational modelling. First, 90 UK-based adults rated the likelihood of various outcomes given various situations. Then 49 others saw each situation and outcome, and verbally gave their best explanations for what happened. People generate a range of explanations for even incongruous behaviour. We present an expanded version of a counterfactual effect size model which uses innovative features (crowdsourced parameters and free text responses) that not only can generalise to human situations and handle a range of surprising behaviours, but also performs better than the existing model it is based on.