It is possible that a maggot did get into narrow slot that the former and coil move back and forth in. It would have to be cleaned out. Chances are it is something else. More than one speaker being afflicted with such a rare problem is a bit odd. Although maggots do like cheese and if the other guy was playing some cheesy stuff, you never know.
Below is a cut away section of a speaker. Below that is an illustrated diagram showing the various parts that make up the speaker. As you can see space is tight in the air gap. Any misalignment can result in impeding the movement of the coil -former-cone assembly.
If you tested the speaker by itself, disconnect from the wiring harness, with an ohm meter, you can tell if the coil is blown. Sometimes when it does this, the coil wires can get jammed in the air gap and lock the cone in place.