Hopkins runs huge medical and life sciences research programs, heavily dependent on federal monies, so this is not surprising.  The University of Chicago (which gets much less NIH money) has not announced a hiring freeze, but has announced a freeze on salaries, the second time in five years.  Thanks Bob Zimmer (the President who ruined the university's finances)!

UPDATE:  Steve Gross shared the message from the Johns Hopkins leadership, which described some of the effects the Trump war on universities is having:

As many of you know first-hand, we continue to experience a steady stream of research grant terminations, suspensions, and delays. We shared in March the loss of more than $800 million at Hopkins from USAID grant terminations. Since January, we also have had 90 grants terminated by other agencies, resulting in the loss of more than $50 million in federal research funding, with more terminations arriving nearly every week. 

Moreover, we are seeing a marked decline in the pipeline of new federal research awards at Hopkins, down by nearly two-thirds since January, compared to the same period last year, despite continued high scores and an increase in submissions by our researchers. We fear that this downward trend may be laying the groundwork for deep cuts to the extramural research programs at the NIH, NSF, DOD and DOE—further fraying the extraordinary and longstanding research partnership between universities and the federal government and significantly curtailing Hopkins' capacity to undertake our core academic and research mission and to sustain the people who allow us to realize it. 

The salary freeze applies only to those earning more than $80,000, and the leadership indicates that they will reevaluate the financial situation come January 2026.