First, let's be clear, they are called terminal because they happen to come from the end of the stem - the terminus - not because they are usually a terminal condition as in causing death. They aren't. Most healthy Phals will eventually develop a basal keiki and keep right on growing. Some will also bloom from more typical lateral spikes even after the terminal spike is bloomed out. Death often comes because people are impatient and toss the plant out when it hasn't grown a new crown in 6 months.

Next, let's also be clear, terminal spikes also occur in wild Phals growing in their natural habitats, not just over-bred, over-cloned or hormonally manipulated hybrids. It is a natural part of how Phals grow sometimes. Asking what causes them is a little like asking what causes that one freckle on your arm when all the others are on your face. Biological developmental processes are complex interactions of many factors, and none are ever completely predictable. Terminal spikes are caused by a Phal being a Phal in response to the environment and its own genetic programming. Sometimes when it is time to send out a spike it starts at the terminal meristem instead of a lateral meristem that may only be a few millimeters away. The likelihood may be promoted by certain conditions, but there isn't anything that inevitably causes them.