Why do pilot and GPWS approach callouts say “minimums” instead of “minima”? A safe instrument approach requires a number of callouts along the way; two of the most important come fairly late in the approach, first to alert the pilot flying that the aircraft is approaching the minimum descent altitude (MDA) for that approach, and then when the aircraft actually reaches the MDA (these callouts were traditionally made by the pilot monitoring, but are now usually made by the aircraft’s GPWS).  At this point, if the pilots have the runway environment clearly and consistently in sight, they can continue and land; otherwise, an immediate missed approach is mandatory.

These callouts are typically annunciated in plural form, as, respectively, “approaching minimums” and “minimums”; however, the plural of “minimum” is not “minimums”, but, rather, “minima”, which is also shorter and takes less time to say than “minimums” (and good time management is one of the many many things that are critical during an instrument approach, especially one in low visibility or over or near obstructed terrain), so why do these pilot and GPWS callouts typically use the longer and ungrammatical “minimums”?
Minimums is an acceptable plural of Minimum (along with minima). 

As for why its used as a call out is likely because it is the preferred pluralization used by the FAA and in my experience the term generally used when talking about "approach minimums" or "weather minimums". 

Its worth noting that the FAA uses both minima and minimums in their handbook.