Momentum in EvE is huge. Alliances live and die because they offer their members a significant enhancement to their EvE experience… or they don’t. When things are going badly for an alliance, whether its leadership, luck, or just being plan outmanned, it tends to spiral. People don’t log in, or join fleets. They may leave their corps, or whole corps may leave.
But it doesn’t take much to swing things. A big fight that you win, an awesome kill, it energizes people, they log in more, they join ops, and you can play that momentum into bigger and better things. On the other hand when things are going great and you hit a pothole, things start to get bad, even a shrot run of bad luck and leadership gets taxed to do more and more, and usually they stop trying as hard after a while, because its a game, its supposed to be fun, when it stops being fun leadership will do less and less and do other things for their fun, which leads to death spirals and failcascades.
Larger alliances and Corps are more insulated from the negative affects (everyone likes being on the winning side), but smaller corps can quickly find themselves in a situation where they suffer unsustainable losses in player hours and even players.