Applying for multiple loans will not necessarily cause your credit score to decrease. However, if you are approved for multiple loans, it may be a sign to lenders that you're in financial trouble. This could cause your credit score to go down.
Having multiple credit inquiries can also cause your credit score to go down. So, if you're shopping around for a loan, it's best to do it within a short period. Then the inquiries will only show up as one on your credit report.