When two actions happen simultaneously in the past, they often share the same tense. Here, "was going on" (past continuous) is paired with "began" (past simple) to show an overlapping event.

১. "Was began" is grammatically incorrect because "begin" is an intransitive verb in this context.
২. "Had begun" would imply the rain started before the harvesting, which contradicts "even as".
৩. Proper tense coordination is crucial for maintaining the logical flow of a narrative.

অতিরিক্ত তথ্য: The word "even as" functions as a conjunction meaning "at the same time as".