Hi.. My legion's training count suddenly reduced by 700 (Cavalry / Footmen / Archers). What could possibly..

Most likely you have some research that increases your training count during Clash of Province Events and you are now in a Warm up week for CoP (your CoP season has just finished) On Mondays during the warm up week there is no CoP event and your boost to training will not be active. When the CoP event starts again you will get the higher count back.

In NON COP days it will drop.(I. E )after every 3weeks of SVS, there is a preparation week on that week only on FIRST day(MONDAY) it will be reduced. Once warmup starts on TUESDAY it will become as usual what you already have... It reduces somewhere around 20%.