IRS investigation

We filed and it was ‘accepted’ then we get a letter from the IRS saying they were reviewing our tax filing because of our income.  Our income had decreased.  We both receive an income that does not get taxed; it’s from a retirement/pension account.  We had the same filing last year using the same tax documents but my husband worked more.  Does anyone know why the IRS would be investigating us?  We have to wait 60 days to hear back from the IRS and can not contact them.