taken from stackexchange.com with 84 upvotez:
"With "bitcoin days destroyed", the idea is instead to give more weight to coins which haven't been spent in a while. [ ... ] This is believed to give a better indication of how much real economic activity is occurring on the bitcoin network. [ ... ] The graph of overall bitcoin days destroyed is believed to show that the genuine level of activity in the Bitcoin economy is continually increasing [ ... ]"
Well, just looking at those huge spikes we can tell that the graph does
not show "genuine level of activity in the Bitcoin economy".
The explanation given by other posters for the last spike, in particular, shows why: "coins moved" does not mean "coins changed hands". That goes for "old" coins as well for "new" ones.