Spikes induced on PCB. EMC problem?
While you have freewheeling diodes on the relay coils, you don't have any kind of spike attenuation on the relay contacts. The contacts need transient suppression for the same reason that the coil driving transistors do.

The inductance of whatever load you are switching can cause a large (several kilovolts) voltage transient with a very fast rise time when the relay contact opens. The transients will then couple capacitively and inductively to the rest of the circuit (and over time destroy the relays). These spikes are sporadic in nature, as their magnitude varies with the amount of current that happens to flow at the instant the contacts open. You will sometimes see multiple smaller spikes when a relay closes, caused by the contacts bouncing a few times before finally settling.  

To suppress these spikes, there are several options that can be either used alone or combined together for more difficult cases:  


A RC snubber network won't eliminate the transient, but even with a rather large resistance it will significantly reduce the voltage rise time, greatly reducing the radiated and capacitively coupled EMF. It has the drawback of passing a slight amount of AC current even when the contact is open.





A bidirectional transient voltage suppressor diode is designed to start conducting significant current once the voltage over it reaches a certain treshold. They activate extremely fast (typically in the picosecond range), but they get costly when high transient energies are to be dissipated, and ratings above 500V are rare. Example: P6KE440CA  
Metal oxide varistors are available for higher power and voltage ratings than TVS diodes, but they are inferior in response time (nanosecond range). They also have the drawback of having a lifetime limited to a few thousand activations, and they like to fail shorted, requiring external protection. Example datasheet  
Gas discharge tubes contain a gas which ionizes when the voltage reaches a certain level, allowing the electrodes within the device to arc to each other. They are only available for large currents and voltages, and degrade over time.  
Synchronous switching. Since you are switching AC, the current crosses zero 100 or 120 times each second. If you manage to time the relay actuation just right, you can (theoretically) open the relay with no current flowing trough it. You would need a mains voltage zero crossing detector and significantly more complex programming to pull it off, and it would only be possible with predictable and consistent relay closing and opening delays. The likely phase shift between voltage and current should also be taken into consideration.