Answer: 

There can be any number of reasons why acceleration values may be excessively high. Consider some of these reasons:

1. High acceleration results values may be correct because, all the spectrums in different directions are combined together for the final response.
2. If enough model mass was captured during modal analysis, consider unchecking / disabling Missing Mass from Dynamic analysis. This may reduce the accelerations results. 
   1. It is possible that missing mass implementation maybe highly sensitivity to rigid supports
3. Although not guaranteed, there is a chance that the model might be corrupted. To rule this out, round trip the model by following the instructions outlined here and compare the before and after results using an software application that highlights all of the changes in a text file (example: Beyond Compare, UltraCmpare, etc..). If the comparison are reasonably close, ignore this reasoning. If the comparisons are vastly different, this maybe a big contributor. Recommend, keeping the round trip model to replace the original model.