Sometimes pickups can have loose parts, or the mounting method isn't secure enough. When either of these things happen, it can lead to unpleasant microphonics (squealing). Most pickups are submerged in wax to hold the innards together (wax potted), but sometimes the wax bonds break down.
A small foam pad placed under a humbucker to pad the bottom of the pickup from vibrating can alleviate some of the problem of vibrating parts of a pickup by providing some upwards pressure. This helps to absorb the vibrations, preventing your guitar sound from feeding back into the pickup.