iptables: ports leaked, but why?

My Raspbian Rpi4 for some strange reason, began to filter external access to ports as seen here:

(hendry@t480s ~)$ sudo nmap 192.168.1.4
Starting Nmap 7.80 ( https://nmap.org ) at 2019-11-10 08:35 +08
Nmap scan report for 192.168.1.4
Host is up (0.0027s latency).
Not shown: 994 closed ports
PORT     STATE    SERVICE
22/tcp   open     ssh
53/tcp   open     domain
80/tcp   filtered http
443/tcp  filtered https
8080/tcp filtered http-proxy
9000/tcp filtered cslistener
MAC Address: DC:A6:32:0E:A7:25 (Raspberry Pi Trading)

I proceeded to restart the device and the problem disappeared:

Nmap done: 1 IP address (1 host up) scanned in 1.57 seconds
(hendry@t480s ~)$ sudo nmap 192.168.1.4
Starting Nmap 7.80 ( https://nmap.org ) at 2019-11-10 08:39 +08
Nmap scan report for 192.168.1.4
Host is up (0.0016s latency).
Not shown: 994 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
53/tcp   open  domain
80/tcp   open  http
443/tcp  open  https
8080/tcp open  http-proxy
9000/tcp open  cslistener
MAC Address: DC:A6:32:0E:A7:25 (Raspberry Pi Trading)

Nmap done: 1 IP address (1 host up) scanned in 0.36 seconds

I did not knowingly change the iptables. Any idea why this could have happened?