The unexpected order of acidic strength of the trihalides of boron can best be explained by

The order of acidic strength of boron trihalides can be determined on the basis of back-bonding. Due to back-bonding, the electron-density is sent back to the boron atom by the filled orbitals. As a result, the Lewis acidity on boron atoms is reduced. But in case of congeners with ligand orbital 3p, 4p, etc i.e. the orbital overlap is not that much good. In such cases the extent of back-bonding decreases. Finally, the decrease in extent of back-bonding results in increase in Lewis acidity.

Thus, the order of acidic strength of boron trihalides i.e. BR₃ (where R is a halogen) is determined on the basis of back-bonding. Back-bonding is a type of resonance which increases stability. Back-bonding is responsible for reducing Lewis acidity on boron atoms.