Spikes are mostly quick research tasks aimed at establishing the feasibility or prototyping of some idea. Thus, they are structured so that the tasks either finish-fast or fail-fast.
However, when a dozen or so spikes are added to the current sprint, there is a possibility that several of them will fail-fast altogether. Let's say there are 4 possible ways of doing something, and the task is to choose the most suitable one and come up with a prototype. If the first attempt fails then try the next possible way, and so on. If the first pick is already a clear winner, then the next 3 aren't necessary anymore.
So, when the current sprint consists of mostly spikes, does it make sense to also shorten the sprint time to maybe a few days, so that reprioritization can happen more frequently?