I must admit that I have never really understood this notion of "activist judges". Surely the very act of judging is in and of itself an "active" action: whether they rule for or against something. 
But I want to understand what this term actually means. This I think is the argument that conservatives, and in some cases liberals use when them deploy the term.

I suppose it has something to do with the idea that judges "should not impose their beliefs willy-nilly upon a democratic nation." The people should decide.  The idea that a court of 7 can overturn what the "populous" thinks is abhorrent. Who after all, are they (the 7) to decide? 
pause. one moment. Superficially, this (might) make(s) some sense; but if you examine it surely one can begin to see it fall apart at the very seams. 
First:
This argument  seems to presuppose the idea that the "mass" is always right, that democracy is the be -all and end-all of a social and political system.  
But America is not a democracy. It's a republic. We elect people who supposedly represent our beliefs/ morals/ convictions &c... They make decisions for us. Obviously they are going to make "bad" (in our opinion) decisions. If enough people believe that they have made "bad" decisions, come the next election the officer will pack up and go home.  (in theory)
Furthermore, how can one ever know what the populous wants. Yes there are statistics, but for anyone who has watched Yes Prime Minister, you can word a survey to make them believe what ever you want. 
So who do we turn to? We are only left with the opportunity of looking to the elected officials. Through the vicissitudes of political parties' fates, the country trudges ever onward. 
But we are now veering off in the direction of political theory... let us try to return to the question at hand: activist judges. 
Second: 
The notion that a judge "overturns" the status quo (however that is to be discovered) is assume that the job of a judge is to enforce the status quo rather than to uphold justice. Clearly, it is ludicrous to assume that judges solely exist to legitimize a part (be it the minority or the majority) of  the public sentiment. 
They are meant to uphold and interpret the laws at hand. Do these laws, which vicariously the populous has approved, obey some greater law (justice), or are they incongruous with it?    That is the simple quiddity of judges, surely. 
Question: 
Does a judge ever impose his personal belief when making  ruling? 
Another more natural question would be: when does s/he not? 
It seems impossible that a judge would be expected to separate his/her moral beliefs with his/her notion of justice. When a judge decides gay marriage is okay, s/he surely imposes eo ipso his own interpretation of justice. Likewise when a judge decides the opposite, s/he does the same. 
I do not want to suggest that justice is either some mountain which can easily be understood, rather it is a question of  interpretation. Dare I say active interpretation. 
So:
Was this decision to allow gay marriage "just"? I suppose I could now marry my boyfriend if I wanted to, so for me it is. For other people, it represents the start of the end of marriage as we know it. 
I don't know which is better. Who am I to say? 
So coming back to our original question: what makes a judge an activist judge? It seems to me that a judge is an activist, and has to be. I think it would be silly to say that a judge ruled for this decision simply because he wanted to jam some "homosexual agenda" down the throats of the already gagging conservatives. 
I would like to think that it was a call to justice. 
But courts have been wrong before, maybe this one will be also... time shall tell    -.l.b.