The main reason are the economic policies most countries around the world adopted.
The idea was basically to increase spending (and money printing) during lockdowns so they could avoid a recession. So they did, and now the bill is coming.
More money in circulation = more inflation
It's not today that money has been printed and it's not all countries of the world that are printing money at the same time, this proves that the reason for global inflation is beyond this, it is a global issue. The US, for example, had printed money in the past and did not trigger hyperinflation since there are measures.
And yes, policies and careless decisions cause inflation, still, it's not all the time that this is happening in countries and still does not trigger global inflation in most cases.
As for me, what causes this present global inflation is too much dependence of some countries on other countries for goods and services and the war between Russia and Ukraine. In such a setup, when something goes wrong in those contributory countries, it has a chain reaction in the dependent countries as we see it now.
Mind you, no policy is perfect and there are always good reasons for them. And the policies that worked this time might boomerang without much reason for it at times, nonetheless, the effects stay in the country most times, not what we experience globally now.