So...because James is mentioned in a (lengthy) passage that says this :
And certain men which came down from Judaea taught the brethren, and said, Except ye be circumcised after the manner of Moses, ye cannot be saved.
Acts 15:1
...and this...
But there rose up certain of the sect of the Pharisees which believed, saying, That it was needful to circumcise them, and to command them to keep the law of Moses.
Acts 15:5
...that means he's a Judaizer? Notice a couple of things :
1. Immediately following verse 1 above, verse 2 says this :
When therefore Paul and Barnabas had no small dissension and disputation with them, they determined that Paul and Barnabas, and certain other of them, should go up to Jerusalem unto the apostles and elders about this question.
Acts 15:2
i) This was a huge deal with Paul (and Barnabas), not some minor issue. As we see later when Paul tells the Galatians about his conflict with Peter (Paul "withstood him to the face because he was to be blamed"), Paul is not shy about confronting those who he sees as wrong. But do we see him confronting James? I don't ever recall seeing that anywhere.
ii) Which leads to the second thought. This disputation was being settled by going to Jerusalem where the apostles and elders are. So as we see later, James is a part of the group that is going to settle this dispute - not feed it.
iii) Jumping the gun a bit, but assuming James is part of the problem again becomes an issue since we would expect he and Paul to go at it....
2. More explicitly, James concludes his response to the controversy this way :
Wherefore my sentence is, that we trouble not them, which from among the Gentiles are turned to God:
But that we write unto them, that they abstain from pollutions of idols, and from fornication, and from things strangled, and from blood.
Acts 15:19-20
i) What is abundantly clear is that James believes Gentiles are to be saved and that they do not need to follow the law to be saved.
ii) And what is the response that goes out from Jerusalem to those mentioned in verse 1?
For it seemed good to the Holy Ghost, and to us, to lay upon you no greater burden than these necessary things;
That ye abstain from meats offered to idols, and from blood, and from things strangled, and from fornication: from which if ye keep yourselves, ye shall do well. Fare ye well.
Acts 15:28-29
The very same thing James himself said in the verses 19 and 20!
So somehow, out of that, James is Judaizing when his message has the approval even of Paul?