What it boils down to if you are using a shared file location is that different application processes are trying to add and delete the same files as cache dependencies.  So webserver1 and webserver2 both use the same shared file location, and if the cache needs to be updated from one server because of expiration, then chances are good that it will also need it on the other.  In a high load environment, this leads to contention.  The higher the load, and the more web front-ends you have, the higher the contention.

One alternative would be to let all the web servers use their own cache folders.  The disadvantage to this is if someone updates on one server and the cache needs to be invalidated, then it would only happen on the one server, but in reality if you put a short enough expiration on each server then they will all only be out of sync for that short amount of time at the most and it will probably not be noticeable to the end user if you put new content up and it is not showing on all the servers at the same exact time.

Another alternative would be to change the code so that only one server does the deletes, and make that server is the only one that updates are done on.