I would be willing to bet the price increase was demanded from Sennheiser. I am sure the retailers of the 650 were pissed that the 6XX sold 5,000 units just in time for Christmas, the largest shopping season by far. At $199, it truly was a stupid cheap deal that anyone would jump at, if they know anything about headphones. At $249, it is still a nice deal, with the caveat that you have to wait (a very long time with this particular drop). It wouldn't shock me if Sennheiser gets the order built and shipped earlier than the expected ship date.
I agree that the 6 month wait may not be worth it to others. For ~$50-70, you can order the HD650 now and enjoy it in a few days. For some this isn't a huge deal and that is what they will do. For others on a more strict budget, the $249 price is worth waiting for. IMO, this isn't really that spectacular a headphone, and there are other models in the $250-300 range that I would go with before the HD650/HD6XX. The Monoprice Monolith M1060 at $299, the Hifiman 400i ($249 on sale frequently). These 2 phones as an example, don't require a special amp to get the most out of them. Sure they will scale well with better source gear, as most headphones do. The HD650/6XX I have read time and time again, need a pretty good amp to get anything close to good sound out of. a decent OTL tube amp like the Bottlehead Crack (which you have to build yourself or pay extra to have them build it), is considered one of the better amps for the HD650. That is another $300 for the base model, and $414 if you want the Speedball upgrade (sometimes this amp goes on sale), which I have read is very much worth the additional money. So to get a great Headphone/amp combo, that is some serious coin.
I would read up on the HD650 here. You'll probably see a lot of suggestions saying that XXX amp isn't good enough and that to get the most out of the HD650, you need amp XXX. Yes the 650 is a classic and well known headphone (I would say it is THE headphone that most other headphones are compared to, really because most people are familiar with it). If you are interested in the 6XX/650, I would highly suggest researching it, and the threads here are a great place to do that.