Your Computer or Network may be Sending Automated Queries.
# Read this notes very carefully,which gives a more detail about what you look for it:
* The Problem is related to the network. You are connected, not necessarily the single computer or phone you are using to search.Google is simply seeing a large number of traffic coming from a single IP Address.
* If your computer or phone is free of malware and you are not engaged in high volume search activity,
another computer or phone connected through the same network might be to blame.Even a large no of devices on a network using Google Search at the same time could trigger the message.
* Google watch as a network that is a single IP Address could be anything from a single devices to a
large network such as aVPN or proxy server. Google has no way of knowing how many devices are sharing the same public IP Address.
* Because the number of global IPv4 addresses is virtually exhausted, many Internet Service Providers assign the same IP address for a number of customers, using their own equipment to keep track of each customer internet traffic.
* If you getting problem then you need to report the issue to your network administrator.This could be the person who look after your home network or work network, the organization whose public network you are connected to, or the company who provides your home or work internet connection(ISP).
* A network administrator is in a position to investigate the cause of such issues and take appropriate 
  action, such as tracking the issue back to a single computer, or switching your connection to a different
  IP address.
* If you are using a home internet connection and your internet service provider allocates your IP address dynamically, the solution might be as simple as rebooting your modem to get a different IP address.
* Blocking could be related to usage of a VPN browser plugin, or program. Try disabling the VPN and see if that helps.When the abuse hitting Google Network stops, Google automatically stop blocking the IP(s)ISP(s) that were sending the bad traffic.

Additional notes for Network Administrators

A reCAPTCHA may appear for network users when Google automatically detects requests coming from a computer network which appear to be in violation of the Terms of Service. The block will expire shortly after those requests stop. In the meantime, solving the reCAPTCHA will let a user continue to use Google services by setting a browser cookie (if a user continues to receive the CAPTCHA page, clearing cookies may help).
Sometimes users may be asked to solve the reCAPTCHA if they are using advanced terms that robots are known to use, or sending requests very quickly.
Use of VPN programs or plug-ins may also be to blame.
It is suggested that you check that proxy servers or group policies have not been set to re-write the google search URL using an older string. Try turning off any URL re-writing and see if this helps.

Just one single "rogue" device on a network is sufficient to trigger the reCAPTCHA for the whole network, so you'll need to monitor the network traffic to identify which device may be the source of the unusual traffic. Of particular concern would be privately owned devices, if they’re permitted to connect to the network, but whose software is not managed by the network administrator.

Solution:

# Customers have also edited their ipv6 entry in their /etc/hosts/ file to add the following:

2607:f8b0:4005:801::200e google.com
2607:f8b0:4005:801::200e www.google.com
2607:f8b0:4007:805::100f scholar.google.cn
2607:f8b0:4007:805::100f scholar.google.com
2607:f8b0:4007:805::100f scholar.google.com.hk
2607:f8b0:4007:805::100f scholar.l.google.com