Complex sentence: When contrasts are not orthogonal, they are not testing completing different aspects of the data, the results of one contrast can then influence the results of other contrasts, this can increase the chance of falsely rejecting a true null hypothesis.
Split into simpler sentences: When contrasts are not orthogonal, they are not testing completing different aspects of the data, the results of one contrast can then influence the results of other contrasts. This can increase the chance of falsely rejecting a true null hypothesis.