Employment is capricious in most situations.

Large and small organizations can both have politics, can see drastic restructuring when business changes or a major client makes a change (drops a product, demands a change, ...). Large companies may insulate somewhat against capricious changes, though non-operational circumstances (particularly mismanagement or liquidity crunches of the holding corporation) can cause sudden and drastic changes (Borders, UAL, American Airlines). Small companies can have cash-flow issues as well.

There's very little that's assured in life. Death and taxes you can count on.