You can grab those graphs from the website camelcamelcamel.com as well. The prices on Grados are unrealistic; Grado headphones have fixed MAPs and will only sell at SRP except for the used market. I've never seen them go on sale, even when they're doing model trade-outs.
Best I can figure on Amazon's prices has to do with availability and their MI abilities. The Denon headphones universally sky-rocketed recently, but seem to be coming back down. Other parts seem to be dancing around, the ones with the wildest flucations are usually also not mentioned as frequently on Head-Fi or other sources (you think those are dramatic, look up the Beyerdynamic T70 (this has bounced between around $399 and around $630), Audio-Technica ATH-W5000 (between $600 and $1100 and back again a few times) or W1000X, stuff like that).
Overall, their prices seem to go in seasonal waves - the best times to shop are before and after the major holiday season (so from September through early November, and around now); the worst times seem to be the major holiday season and early summer. I may be mistaken, or this may not apply to all items, it's just the trend I've seen.
The upside is that Amazon only seems to do this nonsense with some items, other items (like music and DVD purchases) are more stable.