Ta-form doesn't always indicate past tense.
What expressions are used with ta-form?
Ta toki, ta koto ga aru, ta bakari, ta tokoro, ta mama, ta ra, and so on.
Basically, a verb in ta-form means that the action of the verb is completed, but it doesn’t always indicate past tense. Here are some examples:
maiasa kaisha ni tsuita (A) ato de, asagohan o taberu (B). (Every morning, I have breakfast after I arrive at my office.)
kino asagohan o tabeta (A) ato de, gym ni itta (B). (Yesterday, I went to the gym after I had breakfast.)
When action A (“kaisha ni tsuku” or “asagohan o taberu”) is completed, action B (“asagohan o taberu” or “gym ni iku”) is done.
Verb B changes its tense depending on context, but verb A always takes ta-form because it has a sense of completion. Particularly, even though the actions of sentence 1 are not in the past, “tsuita” (ta-form) is required.
The same rule applies to “ta toki”.
Watashi wa itsumo tsukareta toki, massage ni iku. (I always go get a massage when I get tired.)
The whole sentence is in the present, but “tsukareta” is the ta-form. This expresses that I go get a massage after getting tired. Given the above chronological order, you must indicate the completion of “becoming tired”.
This works for “ta ra” in the same way.
Soko o migi ni magatta (A) ra, eki ga aru (B). (If you turn right there, there is a station.)
Ame ga futta (A) ra, ie de eiga o miyou (B). (If it rains, let’s watch a movie at home.)
Kyoto ni itta (A) ra, kireina niwa ga mitai (B). (When I go to Kyoto, I want to see a pretty garden.)
Yamada san ga kita (A) ra, Tanaka san wa kaeru desho (B). (If Yamada comes, Tanaka will go home.) Takarakuji ni atatta (A) ra, nani o kaimasu (B) ka? (If you win the lottery, what would you buy?)
Eki ni tsuita (A) ra, anata ni denwa o shimasu (B). (When I get to the station, I will call you.)
The first verbs (A) are all ta-form. After those actions are done, the second verbs (B) are conducted or occur. In other words, verbs B follow verbs A in terms of time. Many people wonder why if-sentences require ta-form (AKA past form). I hope this explains why.