Well check if there is any SIM registration / big franchise or sale center center there, normal practice they use is to insert the sim in a device and check if it registers to the network.
Check for their hours of opperation and coorelate with your stats.
Or check if any of the Car Tracker firms are using your network as the normal practice is to block their sims for non-payment but the sim still tries to register to the network when SMSing their coordinates and fails. this generates Large LU requests and failures. any similar activity may be responcible