The alarm is that bond yields are more or less `yield = (k / economic confidence)` - if people are confident then they don't buy bonds, so the price of bonds falls, so the yield (which is annualized return) rises. (Thus recessions lead to falling interest rates as people seek safe harbors for money, making credit cheaper.)

What this means is that the bond market thinks available bond yields for cash-on-hand will be lower in three years than five, so they're willing to take a hit now to lock in the longer-term payout. That reflects serious economic pessimism.

It's not a cause for alarm per se, but the inverted yield curve means that the bond market is feeling pessimistic. Rate inversions often happen before recessions, but they're not a fool-proof indicator.