Hey japinard,
have you checked your balance overview and noticed where exactly changes happened? Did your income/taxes drop or did any expenses increase?
Maybe it's just a coincidence with the commuter piers: Often income fluctuates when certain needs of your citizens aren't fulfilled. A new shipment of rum, for example, might not last long enough till the next one arrives, leading to tax-incoming dropping. Or you just barely produce enough goulash and during tempoary shortages people are unhappy and pay you less. Then income increases when some supplies arrive etc etc
Newspaper articles can also increase/descrease income.