This appears to be a situation exacerbated by poor communication between the parties. While I don't know the precise details of this situation, it is not uncommon to need/desire additional information from the applicant if the applicant is being nominated for university-wide fellowships. Those university-wide fellowships require not just the applicant's materials, but also a letter from the department to nominate the student. The letter usually includes a summary of why the applicant is worthy of the fellowship (providing context for the specific department's applicant pool) and how likely the applicant is to accept an offer. This latter information is important, as the university usually over-commits on fellowships, just like departments over-comment on graduate admissions offers. Since the graduate school has even less context than the department does, it is important to indicate a reasonable likelihood of whether the student is likely to accept. Thus, contacting the student and inquiring about their "interest" in the school is not uncommon. This allows us (the department) to write things like "Professor so-and-so contacted happy-prospective-student to discuss our graduate program on February X. While happy-prospective-student has several offers, we believe his/her good match to our program, particularly his/her interest in Prof interesting-subject's research area, makes it likely that he/she will accept our offer." Conversely, if happy-prospective-student is actually I-don't-have-time-for-your-not-Ivy-student, we may choose to nominate someone else for the fellowship. None of this violates the April 15 deadline since this is not a requirement to say "yes, I will definitely attend your institution, particularly if you give me this really nice fellowship offer" in order to be nominated for the fellowship.
Given that this is standard operating procedures at most universities, it should not have raised a red flag at the other institutions. However, there also was no legitimate reason for mentioning this situation to the other schools. Their admission processes will proceed at their own pace. You can send an inquiry regarding status of the process, but no one is going to provide notification until they are ready to provide notification. So there really was no need to contact those schools at this point. The answer to the first school should have been "I am very interested in your program, but I am waiting to hear back from other schools so that I can consider my options fully." (If that is true) or "I am waiting to hear back from other programs, but a fellowship offer would definitely make your school more attractive" (if that is true) or "I am waiting to hear back from other programs. I think your school has a lot to offer, but I have a lot to consider first." All of these leave your options open, but indicate different levels of interest in the program.