The June decision by SCOTUS was weak, and basically invited this exact same thing to happen again.
That's right. The SCOTUS decision was a punt, and virtually worthless, providing no guidance whatsoever on the interaction between nondiscrimination laws in public accommodations and the religious rights of shopowners. Another lawsuit was inevitable - and Jack Phillips likely knew it, since he never removed the statement on his website that his bakery is not now accepting custom orders for wedding cakes.
As for why the SCOTUS punted, it is likely the inability to get Justice Kennedy on board with a substantive ruling. And that may be just as well, because the case lacked the right facts to support a substantive ruling.
But I am on Jack Phillips side this time. In the case that went to the SCOTUS, he refused to bake a wedding cake for a gay couple, before even questioning his customer about the design or message on the cake. But the new kerfluffle doesn't involve a blanket denial of service - the baker objected to the message on the cake (blue icing on a pink cake, in order to promote the customer's message of transgenderism.) That's really no different from a Jewish baker refusing to bake a cake with a swastika on it.
All that the public accommodation law requires is that a customer's business not be rejected merely for who he or she is. It does not compel a shopowner to produce a product with a message with which he disagrees. Jack Phillips was within his rights to refuse the customer's requested message on the cake. Indeed, it appears the customer chose Phillips' store because he/she knew the message would be provocative to him.
See the difference between the two incidents? To me, it's crucial. So my best wishes to Mr. Phillips and his attorneys as he fights the good fight for free speech.