1. The most immediate factor is the French Victory in the Hundreds Year War. This had the effect of pitting the nobility, which have recently had fought in the war, against each other as they blamed each other for the loss. Richard of York for instance, hated the Beauforts for being incompetent and losing Normandy which he fought to defend.

2. Lots of lots of newly unemployed soldiers coming home from France. Which allowed the various barons to hire them into their personal retinues to challenge the crown.

3. Once the political struggle got violent at 1st St.Albans it gets really difficult to stop the cycle of hatred, the sons of the men the Yorkists killed at St.Albans would come back to haunt them at Wakefield. The execution of Edmund of Rutland at Wakefield meant that Rutland's brother Edward IV is a lot more ruthless and so on.

The usage of attainders against losers meant that being on the wrong side left you forfeited of titles and property even if you survived. The savagery of the war meant a lot of the old feudal customs of war (i.e ransoming captured nobles) got thrown out the window.

4. The deposition of Richard II by his cousin Henry of Bolingbroke back in 1399 seriously weakened the legitimacy of the monarchy, from now on any baron with enough Plantagenet blood is a potential claimant. What this meant is that simply killing direct claimants isn't good enough anymore. By all means the house of Lancaster went extinct in 1471 but Henry Tudor was able to press his remote Lancastrian claim by the end.

After all, a lot of the schemers were repeat schemers so it takes some backdrop to allow various nobles to say “sure you failed the last two times and we lost most our men but why don’t we try again.”

Can you think of any specific instance of this?

I think this was only really true of the Lancasterians in 1471, and it's understandable because many of their leaders were attainted. What this meant is that they lost their lands, property and titles. Given this....what do they really have to lose to try again?

The first and most obvious factor to mention would be the Black death that swept across Europe from 1346–53; with many resurgences over the following centuries. The pandemic was unprecedented in its size and scale killing approximately 75-200 million people. Since it struck so unpredictably many of the scions of the great noble houses were destroyed thus disrupting the complex web of alliances and patronage and feudal networks that held the kingdom of England together. The plague hit the peasants less hard than the nobles but there was still a mortality rate of around 30%-50% among the peasantry. Previously agriculture had operated on a purely feudal basis with fields divided into strips farmed by serfs that were tied to the land. As many of them died off that feudal model of agriculture became unsustainable since there were no longer enough people to work the land. The remaining peasants were able to demand more land and higher wages etc. This eroded the profits that landlords made and the trade networks that sustained the towns and powered England's international trade collapsed -- setting the scene for a longer period of unrest in the aftermath of the peasants revolt in 1381.

As mentioned before, the feudal structure that had held society in place since the high middle ages began to collapse and the creation of a centralised, autocratic monarchy began in earnest. It should be noted that this was the time of the unification of Spain under Ferdinand and Isabella. The increase in the power of the king (although it was never as pronounced as in places like Spain and Russia) took some time to work through.

The Hundred Years war was vastly expensive both in terms of manpower and treasure and the sudden reversals after the death of Edward III and the subsequent loss of continental holding pissed the nobility off to no end. Royal authority was weakened and the nobles were more willing to challenge the king since he had failed to stop the French advance (with the obvious exception of Henry V).

Henry VI was a weak king. After the death of Richard II the line of the black prince (the first son of Edward III died out) the son of John of Gaunt the third son, Henry V, seized the throne. He died young and left behind him a baby who was not a year old as king. Had he been ruthless enough and sane enough to destroy the rival claimants to the throne (the yorkists who claimed the throne because they were descended from Lionel of Hainaut, Edward III second son) war might have been averted.Henry's weakness discredited the Lancaster claim to the throne as it was seen by many that Richard Duke of York would make a better king. So the rub of it might be that both sides thought that they had a better claim to the throne than the other, so it made perfect sense to fight over it.