April 15th is a common deadline for graduate school applicants because many graduate schools agree to abide by a Council of Graduate Schools resolution calling for binding commitments to graduate schools offering financial support only on or after April 15th of each year. The resolution has been in place for over 40 years, but often there are schools and individual staff at a university that have never heard of it.
Students should review the resolution to see if the universities they are applying to abide by the resolution (my institution does follow it even though we may not be listed). I believe students should push back on admissions officers asking for commitments before April 15th if the university has signed the resolution. There is no shortage of admissions officers that don’t know their university has signed this resolution.