Deep learning methods have significantly advanced the performance of image matting. However, dataset biases can mislead the matting models to biased behavior. In this paper, we identify the two typical biases in existing matting models, specifically contrast bias and transparency bias, and discuss their origins in matting datasets. To address these biases, we model the image matting task from the perspective of causal inference and identify the root causes of these biases: the confounders. To mitigate the effects of these confounders, we employ causal intervention through backdoor adjustment and introduce a novel model-agnostic cofounder intervened (COIN) matting framework. Extensive experiments across various matting methods and datasets have demonstrated that our COIN framework can significantly diminish such biases, thereby enhancing the performance of existing matting models.