Why are the glaciers advancing and retreating?
There is no simple answer. If a glacier has a steady source of snow turning to ice in the mountains, a good lense of water on which to slide along the bedrock, enough gravity and momentum in downhill movement, a good moraine of rock and rubble at the front to insulate it from water erosion and cold enough temperatures year-around, a glacier will advance. If it loses enough of these, it will retreat.
Is global warming or climate change causing the glaciers to melt?
Another good question that does not have a simple answer. Temperatures do effect glaciers. If there is not enough snow falling and accumulating in the high mountains then turning to ice, the glaciers will retreat. In Glacier Bay many of the glaciers are retreating and have been over the last 200 years. Whether this be attributed to global warming or climate change is uncertain. Keep in mind that there are also a few glaciers in the park that are advancing and a few that are stable.
What is the chance of seeing an ice fall or calving?
Chunks of ice break from the glacier year around. Calving happens regardless of sun or rain, hot or cold, sometimes not at all in your one hour visit, sometimes multiple times in that hour, no one can predict this very dramatic event.