Question: Hello my server is getting hit by spiders from bingbot, google, yahoo, yandex, and some place in the UK all at the same time crashing it.

While I have no reason to think any one person did this, it strikes my odd that it happened all at the same time. 

Should I be worried? 

The only changes before this started was that I changed all the server passwords.

Answer: 
It's crawling because of some link somewhere. You would need either to throttle bots or fix the websites so they are faster. Not sure about how to throttle the bots, you might want to check with google webmaster page or this:

<https://stackoverflow.com/questions/1328436/how-to-prevent-googlebot-from-overwhelming-site>
I disagree that this is always normal behavior. I am under a similar DDOS attack form the search engines. We have ample resources and numerous redundant servers handling our site. All with DNS failover. And BOOM we got hit and knocked off the web. First I saw reports of failovers going up and down like crazy. 

Thought we had a bad AJAX or un-cached query page being hit. As I looked deeper into the logs and saw every bot, pretend bot and wanta be bot from both recognized and unknows, I knew it was an initiated attack. Why? Our robot.txt only allows googlebot, all else are excluded from our server fleet. 

So the robot.txt was being ignored, and we are being brought down. So if you ignore my robot.txt, my firewall will not ignore you. We firewalled (and still are) over 60 spiders so far. (all hitting at the same time, ignoring both the 360 delay and the disallow in our .txt). So I would not always think this is normal anymore.