I know "talk with your players" is old advice, but it's repeated often for a reason.
This really seems like a communication problem between the GM and players. Maybe the players don't understand all their options during combat? Or maybe they don't trust that the GM will let them run away, regroup, etc.
With new groups I will ask them to vote on a danger level of the campaign. 1 means no risk of death, 2 means some risk of death (but I will give them obvious clues when they are entering dangerous situations), and 3 means bring it on.
I'll admit that 3 is my personal preference as a DM, but my last group voted for 2. So if they were in a really dangerous situation, I'd either warn them, or soften the enemy strategies, or provide an obvious line of escape (like the enemy offering to negotiate).