There are actually three arches in this picture.  To the far right is the porthole-like Navajo Arch.  In the left foreground is Landscape Arch, and directly behind it is Partition Arch.

The rib of Landscape Arch was much thicker before September 1, 1991.  That morning, hikers thought they heard cracks of thunder from distant clouds.  Visitors resting under Landscape Arch heard loud cracking and popping noises from overhead and fled as small rocks came tumbling down from the slender 306-foot-long span.  Moments later, a 60-foot-long rock slab peeled away from the arch's right side.  When the dust settled, 180 tons of fresh rock debris lay scattered on the ground.

Unseasonably heavy rains over the prior ten days probably filled the porous sandstone and the extra weight was 'the straw that broke the camel's back'.  The lesson, of course, is that nothing lasts forever.  Eventually, Landscape Arch and every other arch in the park will collapse.  It's only a matter of time, but as we saw at Tunnel Arch, others will form to take their place.