The phrase "simmering tensions" refers to a situation where there is ongoing or growing conflict, anger, or unease that is not openly expressed or resolved, but could potentially escalate in the future. Full definition
The long simmering tension between institutions and the art market has begun to boil over. (artmarketmonitor.com)
But simmering tensions remain at the top level of the coalition government. (newscientist.com)
Long - simmering tensions bubble to the surface when a young woman who has been in and out of rehab returns home for her sister's wedding. (latimes.com)