The possible trademark “squatters” include the agent and distributor of your products, your business partner or competitor, your employee, your trademark agent, your customer or other entity or individual which gets to know your trademark through business communications.
In addition to that, there are local Chinese who every day scan the Internet looking for foreign brands to register as trademarks in China. Some just go to exhibitions and collect brochures and business cards as a source of idea.
They do this because it is very profitable. It costs a local Chinese very little to register a trademark but a foreign company will often pay handsomely to recover it. It is fair to say that recovery of a trademark, if this is possible, will cost at least 10 to 50 times the cost of registering it in the first place.