There are two ways to consider this that initially spring to mind. First, it could be a reflection of the robustness of balance sheets as mentioned, and the high confidence investors have in many of the large companies that have come through the financial crisis and out the other side.
In this scenario, investors could breath easy, as it would provide reason to believe that things will be different this time and a long market fall will not follow an M&A spike, at least in the near term.
A second possible scenario is far less sunny. The stronger equities rally seen this time around has been seen by many as being fuelled by rock-bottom interest rates and quantitative easing, rather than being reflective of company performance and reduced gearing. If you buy into the argument that equities gains have been largely if not completely linked to QE then it is not a huge leap in logic to believe the pattern will repeat itself, and when M&A peaks a sustained sell off will ensue once again.
In this scenario, the steeper the climb, the harder the fall.