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: 
This whole discussion possibly belongs over in Pro Webmasters, however; as pointed out in one of the comments is applicable here as well.

Having several search engines hit your site at once is absolutely normal behavior. If you managed to get links to your site from other reputable sites, you will get indexed. The most brutal are Yahoo, Yandex and Baidu. The first time our website bogged, I went into DDOS mode and got mostly embarrassed by finding every IP was coming from recognized Microsoft, Yahoo and Google sources (verified by IP block ownership, not UA Strings). The answer was to bolster the website resources to handle the load. 

Despite what you surmize about the number of simultaneous users, your webserver must be able to handle Google, Bing and Yahoo simultaneously indexing your site plus your expected traffic. If your site cannot take the load from being indexed, you will need to either exclude or restrict the search engines. While there is this pathetic robots.txt entry that is almost supported, you will have better results by signing up with Google, Bing, Yandex webmaster tools and use their throttling pages to shape their indexing traffic to best fit your website's schedule.

Verify where the traffic is coming from by analyzing your web server access logs. Look up the IPs (http://www.botsvsbrowsers.com/ is one such tool to help identify bot traffic sources). Yandex and Baidu are respectivly Eastern Europe and China. If they're not in your venue, ban them to save your bandwidth.

**EDIT:** After looking through the access logs, do be on the lookout for sudden increases in traffic from a known search engine UA string with odd query strings. User Agent strings are not proof the traffic comes from who they say it comes from as the person sending the traffic can generate them at will and try to hide behind the UA identity.
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>