When and how did the concept of psychological trauma begin to be extended and expanded to include more mundane and less severe experiences? I'm in my first semester studying mental health counseling at a master's level and I have heard several of my professors make comments to the effect that over the past 20-30 years they have seen the definition of trauma shift dramatically to include things of lesser and lesser severity. I was wondering where this trend started and whether there were any particular researchers or organizations that spearheaded this movement of thought within the field? Is it the result of any particular philosophical, political, or social movements that have affected the field as a whole? or have there been any landmark works that have pushed the field of trauma research in this direction?
I've already read The Trauma Spectrum by Robert Scaer, but it seems like things were already headed in the direction of expanding the definition of trauma already.
One can't be certain what that kind of allusion refers to, but there has been a level of controversy stemming from the changes to the trauma criterion in DSM-IV (it was changed again in DSM-5). The change in DSM-IV--which does correspond with the time frame you mention--was seriously lambasted in some corners, e.g.:

Criterion A1, certifies three groups of people as potentially eligible for the diagnosis of PTSD: direct recipients of serious threat or harm, such as rape victims or combat veterans; those experiencing vicarious exposure to the trauma experienced by others, such as bystanders at a drive-by shooting; and those experiencing ‘informational exposure’. This third group is new in DSM-IV. It consists of people who are ‘confronted with’ information about threats to others, such as horrified television viewers of the 9/11 terrorist attacks (Marshall et al., 2007). Although the authors of the original PTSD diagnosis surely would not have envisioned these viewers as trauma victims (McNally &amp; Breslau, 2008), our current DSM concept of trauma certifies them as trauma survivors just as much as it does those who escaped the World Trade Center before its collapse. In fact, one survey indicated that about 4 per cent of American adults living far from the scenes of the terrorist attacks developed apparent PTSD (Schlenger et al., 2002), presumably while watching coverage of the attacks from the comfort of their living rooms. These people developed what Young (2007) wryly calls ‘posttraumatic stress disorder of the virtual kind’ (p.21).
Because of this bracket creep in the definition of trauma (McNally, 2003a), most people today qualify as trauma survivors. For example, in one epidemiological survey, 89.6 per cent of adults in southeastern Michigan were trauma survivors by DSM-IV criteria (Breslau &amp; Kessler, 2001). [...]

On the other hand, there are studies which point to a limited practical impact of that textual change between DSM-III and IV, in terms of actual PTSD diagnoses:

We used data from the 2009 PTSD diagnostic subsample (N=3,013) of the Nurses Health Study II. We asked respondents about exposure to stressful events qualifying under 1) DSM-III, 2) DSM-IV, or 3) not qualifying under DSM Criterion A1. Respondents selected the event they considered worst and reported subsequent PTSD symptoms. Among participants who met all other DSM-IV PTSD criteria, we compared distress, symptom severity, duration, impairment, receipt of professional help, and nine physical, behavioral, and psychiatric sequelae (e.g. physical functioning, unemployment, depression) by precipitating event group. Various assessment tools were used to determine fulfillment of PTSD Criteria B through F and to assess these 14 outcomes.
Participants with PTSD from DSM-III events reported on average 1 more symptom (DSM-III mean=11.8 symptoms, DSM-IV=10.7, non-DSM=10.9) and more often reported symptoms lasted one year or longer compared to participants with PTSD from other groups. However, sequelae of PTSD did not vary systematically with precipitating event type.

In other words, the expanded definition could have possibly been abused, but in practice that didn't happen much.