In this case it's CAT CDMA, but they all have it, TOT, 3BB etc. What exactly is causing this weird fluctuations ?

Depends on what you are doing. Torrents? Speed tests? Browsing? Unless you are downloading a large file from a single source, the graph is not that unusual for net traffic.

The ISP could be using some form of traffic shaping, the network is busy or the source is busy or your machine is busy. Data flow has to be examined on the whole path from source to destination. Any over subscribed or slow segment along the line will cause limitations.

Traceroute on linux or tracert on windows is handy (not perfect) to see traffic bottlenecks along the link. ICMP is not "priority packets" so take it with a grain of salt.

Checking when things are working well and when not then comparing will tell a lot.

In my case Cat CDMA has been going downhill rapidly over the last 6 months with the network being silent (sleeping) about half the time. It becomes incredibly frustrating to have the network continually lockup when online and still have to pay the full bill for the monthly service. What I am experiencing resembles your graph closely with the dropouts averaging 1 to two minutes - I do not see the time scale on the op's graph.