I think Rustynailer probably summed it up, it is most likely due to lower resistance either the fissures are more open now or, as the dramatic photos suggest the surface cracking is leading to a reduction in pressure. If it was a reduction in the volume of magma I think we'd see a much bigger drop in the number of earthquakes, whereas the frequency doesn't seem to have reduced that much just the magnitude. It can't be long now before it erupts, just have to see if it will interact with Askja or not and if the central volcano will play a part or not. Very impressive photos, sort of stuff nightmares are made of, reminds me of that film from the 1950 I think where the earth was going to be split in two unless some mad geologist launched a nuclear missile down a mine shaft, bonkers film very enjoyable though, lots of scary cracks appearing and disasters happening, anyway somewhat off topic there. Will have to see how things are in the morning.
M