Okay, so I don't know much about lore in detail, but there are some things I have read/heard.

One thing is that Lorkhan's heart is red mountain. Another is that Lorkhan previously "hid" parts of the world that Alduin wanted to eat (whatever that means, I don't know how he could just put a part of the world under a pillow or something). And then of course Red mountain exploded in the Infernal City.

My poorly formed theory is that this weakened Lorkhan's life force and Alduin was then free to eat the worldf uninterrupted. Any lore experts wish to tell me whether or not there is any validity in that?

Yep. http://www.imperial-library.info/content/nu-mantia-intercept.

(The necessary thing was the removal of Lorkhan's heart after Morrowind, but I'm sure the explosion of Vvardenfell didn't help either).

Nah I don't think Lorkhan caused Oblivion or even Skyrim.

Dagon caused the events of Oblivion to occur as he's been planning for years to renter and reclaim Tamriel after his defeat at the hands of the Arena's hero.

Skyrim is probably caused by either Akatosh rentering the world after the pact with Alessia was broken when Martin broke the Amulet and became the Avatar of Akatosh. Something out of that event caused a shift in the balance and reawoke a certain Alduin from his slumber or my main theory is that someone found a shrine to Alduin, summoned him by reading the Dragon langange and then Alduin spoke in the Dragon Language reawaking other dragons from sleep where ever they slept and now can proceed on his original goal to eat the world away, whatever that is.

Lorkhans heart was the red tower.
With the red tower gone, the barriers protecting Tamriel were weakened.
The dragonfires where the white tower. When they fell during the Oblivion crisis the barrier was no more and Dagon was free to invade.
The ending of Oblivion fulfilled the pact Akatosh made with men and freed him from its bounds.
So he is free to end this kalpa and begin the next by destroying it as Alduin the world eater.

Each game is caused by certain events in the preceding games. The events of Daggerfall were caused by the political turmoil during Arena. Morrowind was caused by the hero of Arena busting into red mountain to retrieve a piece of the staff of chaos, and the destruction of the dwemer big stompy robot in Daggerfall spurring Dagoth Ur into action. Oblivion was due to the fall of the tribunal in Morrowind and again the destruction of the Numidium in Daggerfall.
The only game to stand completely alone after Arena is Redguard but that is because it's a prequel to the whole series.

One can also if one so chooses think in the form of the Towers being destroyed. Arena, the staff of chaos. Daggerfall, walk-brass tower AKA the Numidium. Morrowind, red mountain and the heart of Lorkhan. Oblivion, the stone of white gold tower, the Red Diamond of the amulet of kings. The towers are for some reason vital to the existence of the world by either binding down the natural laws or allowing them to be changed.

One can also believe that the breaking of the amulet of kings freed Akatosh from his pact with the Cyrodiilians, allowing Akatosh/Alduin do what he believes must be done, meaning destroying the world for the next cycle.