Engagements may be negative during your chosen date range if more people removed engagements from previous posts than added new ones.
This happens if:
Someone has deleted a comment, retweet or removed their like from the post.
Someone deactivated the social account they used to engage with your post.
For example, if your posts from last week had 50 comments, but 10 of those comments were deleted this week, those 10 would be subtracted from this week's engagement total.
Happy tracking!