It is because of the bots/robots. Every website is hosted on a server (PC). Just like your PC, when many processes run at the same time, it will become slow. This site can handle 25 users loading a page at the same time (exact second) and it is rare. Everyone not going to load pages at a same second and it is possible in large websites.
There are 86400 seconds in a day. This site can handle 2160000 page views (86400x25) per day if there are 25 page views every second (also every page has to be small enough to load in a second). A website will not only attract visitors but also the bots. There are good and bad bots. Google bots, Bing, FB bots will crawl the web sites to display on their service. We have to configure, that Google bot and other bots to have a delay when crawling each page. So they won't crawl all pages of the site at the exact second. The bad bots will ignore such configuration on the website.
The bad bots are used to post spam message, links, gather Email address and other details from publicly visible web pages. For posting messages, they first try to create an account and there might be a captcha. They try solving them. These will affect the server since many processes running at the same time. So, it will stop all its services and will start showing the error message for a while and return to its work. There were many such bots tried to register on the site and it caused the error.
Large sites like Google uses multiple lighting speed servers in different locations. You will be connected to the nearby server and if the server is already handling many tasks then your request will be processed by another nearby server to your location.
There are some ways to reduce the overload of the server. 1) Just like your PC, upgrading the RAM, processor speed and other specification will make a server to handle more number processes to run at the same time. 2) Finding ways to reduces the web pages size by reducing the code will also help. 3) Using services like CloudFlare (which saves information from the original server and displays them to the end user from its own server) will reduce the server load. Input data has to go to the original server through. 4) Using cache, It will store the common elements from the web page to the user browser and so it won't have to load every time. 5) Google has introduced AMP HTML, it has many code and design limitations. When using the AMP HTML, the page size will reduce and also it limits many designing properties of the web page. It is more suitable for news sites and blogs. Not so good for forums and Q/A sites.
You can watch a interesting video below.