Ice sheets move. As new snow falls in the elevations it compresses into ice and gravity carries it downhill.
Slowly, but inexorably.
The end of the ice sheets stick out over water. Ice has weight, even though it can float. When it extends to a point where enough of it extends past land the weight can contribute to it, quit literally, snapping off.
Warmer temperatures can and do contribute to and potentially aggravate this process, but the basic mechanics are above.
As for what happens, the new iceberg drifts, melts, breaks up, drifts more and eventually melts.
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