...so how did the incursions began? what was the event that

They're triggered by writers running out of ideas

>>94654756
Doom. He caused the early cascading collapse of the multiverse to foul the plans of the Beyonders to detonate all the Molecule Men in the multiverse at the same time.

By causing the early death of the multiverse, he forced them to act and expose themselves, so he could kill them all with a time-travel-bomb fuelled by Molecule Men(cfr. "hoisted by their own petard")

>>94654756
Age of Ultron and Spider-man 2099 coming to our timeline. The universe and multiverse couldn't handle the constant strain of everything from time and different universes bumping into each other.

>>94654756
Hackman sucks, literally Bendis was a better Avengers writer

To counteract a reality where the universe died earlier than normal, just make an universe that dies later than normal.
There. I solved the comic book logic problem of this incursion thingy.
Creating universes isn't that much of a problem in a marvel superhero show. They have magical crystals, gods, magic and 4th wall-breaking authors and artists interacting with the cast who can do that easily.