Had the soil been soaked (normally not something to do in other seasons) a day prior to your putting it out for winter? Did you ever dump a handful of snow on the soil over the winter so it could drink a little on melty days? Now, did the weather turn cold again at some point since you brought it out of its protection? That freezing, thawing and refreezing (the latter is a killer) is something to be very careful about.. better to be a little more patient til you're sure all chance of frost is gone. BUT - it's possible the tree was in bad shape before you did anything... because evergreens can look ok for quite a while even after reaching a point of no return and that would not be your fault but the fault of wherever you bought it (Amazon not being the best place to buy bonsai - to them they're commodities, nothing more, and are not going to have been taken care of before you got it). And to be honest, BB is also not a well thought of expert place to get bonsai advice... they just think they are. I suggest that the next time you want to do bonsai, learn more before buying - not from the care tags (they're terrible!) but local clubs, lots of books (no one bk is enough to get a consensus on things and will never cover half the stuff you might need to know). IF, however, you yourself watered the tree a lot for some time prior to putting it outside, that alone could easily have done it in too - junipers like to get quite dry between waterings... they're not houseplants unfortunately.