radioResource config dedicated

Question

There is a scenario where UE is having a default bearer and request for a dedicated bearer (lets say UE want to trigger a Data Call). As per my understanding:

UE sends Service Request + Bearer resource allocation.
After receiving Initial UE MSG .. the MME initiates activation of the default bearer with the S-GW/P-GW by sending the GTPv2 modify bearer request message to the S-GW.
Once MME receives the Modify Bearer response from SGW. MME will send "Bearer Resource command to SGW >>PWG.
Then PGW will trigger Create bearer request for Dedicated bearer establishment.
My query is why MME send modify bearer request to activate default bearer as already the UE was having default bearer.. Please correct me.
Thanks,
Sambit Routray