This will be a very speculative post.
I came across some explanation of the original of the word Tagalog, which is said to come from taga+ilog.
I think the word Tagalog is derived from taga+look. Look means bay, so taga+look means “inhabitants of the bay”. This bay refers to the Balayan Bay in Batangas, which, together with Bombon Lake (now Taal Lake, which was at that time connected to the sea via a channel), was heavily populated, with 9,000 tributarios.
It should be noted that when the Spaniards arrived in the Philippines, it is the Balayan Bay settlements that represented the true Tagalog speaking areas. The bay region up to the channel area in Balayan shores as well in the channel and inside the Bombon Lake has settlements.
When the province of Bombon was created in 1578 and changed later to Balayan Province in 1581, it includes the present territories of Batangas, Mindoro, Marindoque, parts of Laguna and Quezon.
The bay could not have referred to Manila Bay as most of its northern shores speaks Kapampangan, including Tundun (Tondo) down to the Pasig River. The province of Tondo when it was established include Pampanga, Bulacan, and Morong (now Rizal).
Also, proper, pure Tagalog is referred to as “Taal na Tagalog”, which refers to the Tagalog spoken in the Balayan Bay and vicinity.