Acidity of aluminium trihalides
I have chanced upon the answer in an inorganic chemistry textbook (Essential Trends in Inorganic Chemistry by D. M. P. Mingos) I was reading. It turns out that this trend only holds for the boron trihalides and does not apply to the aluminium trihalides. However, the book did not provide the explanation for the exception of boron. 

My thinking is that perhaps, backbonding is not present in the aluminium trihalides, possibly due to incompatibility of the p atomic orbitals for pi bonding between aluminium and the halogen atoms. Does anyone else have any insights to share?