how do you explain a weak GPA?

A few thoughts come to mind:

1) If you are converting your GPA from a foreign system, make sure that your conversion is sound. A naive conversion to a 4.0 system is almost guaranteed to introduce a substantial downward bias (e.g., a 75% percent of a 100% scale is almost certainly better performance than a 3.0 on a 4.0 scale).

2) Compared to other graduate/professional school applicants, MBA candidates tend to have much older undergraduate records. The older the GPA, the more plausible your story that it no longer represents your true performance.

3) Relative to other graduate/professional schools, MBA programs have rather modest GPA expectations. Even schools in the top clusters have a mean not far above a B+.

4) Be careful with "cherry picking" when describing your GPA trend. In other words, if you wish to point to some sort of favorable trend in your GPA, it should make intuitive sense and be easy to explain.

5) As a gross overgeneralization, schools are wary of the "bright but lazy" concept. This can be a risky profile and higher cluster schools have a ready supply of students students with similar overall strength but less risk.

6) "What" and "Where" really do matter. The major and the school that awarded the degree are important considerations when evaluating a student's undergrad performance.