Why does buscar need "qu" in the preterite tense?

Buscar (to look for)
Comunicar (to communicate)
Explicar (to explain)
Pescar (to fish)

RULE: In the preterite tense, the spelling of first person singular (yo) of verbs that end with --car change from "--cé" to "--qué."

Thus, we have: yo busqué, comuniqué, expliqué y pesqué.

In general, the sound /ke/ is spelled "que." Remember the following:

c + a = /ka/
c + e = /se/ qu + e = /ke/
c + i = /si/ qu + i = /ki/
c + o = /ko/
c + u = /ku/

Therefore, this change is due to the general spelling in Spanish. If you want to remember this rule, use the following trick: Cecilia.