An unexpected letter announcing monthly payments, or money arriving monthly before any return has been filed, unsettles people new to the Dutch system. A provisional assessment is an estimate rather than a verdict, and knowing what triggers one explains why the self-employed and those in a first partial year see them so often.
A voorlopige aanslag is the Belastingdienst's (tax office's) estimate of what you'll owe or get back for the current year, issued so you're not settling a full year's tax bill in one lump sum after the fact. It's based on your most recent known figures — last year's return, your employer's payroll data, or numbers you entered yourself when applying for one — and it either bills you in monthly installments or, more often for expats, refunds you money every month in advance (typical if you're eligible for tax credits your employer isn't applying via payroll, or you're claiming mortgage interest deduction). You might get one automatically if the Belastingdienst expects your situation to produce a large under- or over-payment — self-employment income, a second income stream, or a first partial year in the Netherlands are common triggers. None of this is final: once you file the actual return, everything already paid or received under the voorlopige aanslag gets reconciled against the real numbers in the definitieve aanslag (final assessment), and only the difference changes hands.