As part of a long-term program to develop effective, research-based professional development programs for physics graduate student teaching assistants (TAs), we first identify their current classroom practices and why they engage in these practices. In this paper, we identify a set of teaching practices we call "focusing on indicators," which occurs when TAs use signs such as key words or diagrams as evidence that students understand the target idea; these indicators are more superficial than a detailed explanation. Our primary finding is that although the three TAs discussed here share a common behavior, the beliefs and motivations that underlie this behavior vary. We argue that TA professional development focused on changing these TAs' focus-on-indicator behavior is unlikely to be effective. Instead, responsive TA professional development will need to address the TAs' beliefs that guide the observed classroom behavior. (Contains 5 figures, 1 video, and 1 footnote.) [A 50-item list of references and notes is provided.]