Sudden traffic burst lasting only few second is detechted in my old android phone ntp server

I use my old android phone as ntp server with 4G unlimited data connection but with only 2Mbps for both upload and download, using my real time traffic monitor, the traffic mostly remain stable(about 50-100 packet/sec). However, sometimes there is a sudden and dramatic increase in traffic which goes up to over 2000 packets/sec or 1.4Mbps(70% of total bandwidth). Also, the traffic seems coming from many different IPs(but most of them are from China, Hong Kong and Vietnam). Could you explain this phenomenon? My NTP IP is changed to Smartone Hong Kong).

Additional information of server
Operating system: Android Linux 6.0.1
CPU: 4x2.15GHz

You should set up NTP server into the production pool only with long term stable fix IP address. Very likely certain NTP clients with NTP server (ntpd or chrony) running still try to synchronize to the previous IP and they do not receive time information any more:

My IP is always fixed if my phone is not turned off,however,on that day I forgot to charge the phone so it run out of battery and switched off, next time I will be more careful so that my clients won’t be affected

Some clients cache the NTP server address for a long time. The Pool guidance says:

Your computer must have a static IP address and a permanent internet connection. It’s very imporant that your IP address doesn’t change or only does so extremely infrequently (say once a year or less).

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