There are two classes of risks that are typically considered in grant-writing:
- Programmatic risks that the money will be wasted because something doesn't go as you hoped.
- Safety and ethics risks based on the specific contents on your research.
The second is generally covered under a checklist of declarations regarding specific topics (e.g., "human subjects research," "dual-use agents of concern," "vertebrate animal research"), rather than general "risks", so I'm going to write my answer addressing the first class.
With regards to programmatic risks, the big issue here is research always involves the unexpected, most often emerging in the form of things not working as well as you thought they would. Progress might be slow, effects might be small, vital supplies might become unavailable, things just might not work, etc.
What grant reviewers are generally looking for here, then, is evidence of clear thinking about how to handle setbacks of this sort. You really don't want to give money to somebody with no "plan B", and then 3 months into the project something goes wrong and they end up just frittering away the rest of the money banging their head against the same wall.
How I recommend assessing risks, then, is to sit down with a your project plan and asking yourself the following questions:
- For each step in the plan, what materials/people/facilities/information is needed before I can begin the step? What happens if they aren't available as expected?
- For each step in the plan, what does "failure" look like? How many different ways can it fail?
- Which of these problems are most likely to endanger the overall goals of the proposal?
- For all of the "important" problems, what is the plan B for how to ensure money is spent in a worthwhile manner if they are encountered?
Doing this assessment may also cause you to reorganize your plan: sponsors often like to see the most "critical path" or high-risk work up front, so that the most significant and productive problems can be encountered sooner rather than later (and the project terminated if necessary).