Just enforcing the law. Happens all the time https://amp.theguardian.com/technology/2016/may/24/google-of...
French news mentions that the alleged crime is atteinte à la probité or lack of probity. In other words, things like corruption, influence peddling and so on. There are scant few details available, but this might be something more serious than just plain old tax evasion.
Now, because this is Huawei, I don't disagree per se, but that's a different matter. This company should just be outright banned from Europe.