A spike is a sudden increase or decrease in something, usually data that is being monitored in some way. In a game like EverQuest, it can refer to a sudden, massive gain or loss of health (as a result of heavy incoming damage or a powerful heal). If something is spiky, it tends to go up or down in erratic, sudden bursts.
If a character dies as a result of spike damage, it is common to say that that player got spiked.
In recent expansion paks, particularly in Secrets of Faydwer and beyond, group geared players have experienced more than the usual number of difficulties with their tanks getting spiked, as the pace of combat has grown more rapid.