The first question is one I wonder about myself. There is no record of why amiable relations should have reverted to aggressive interaction - best guess would be that Teotihuacan was on the rise. The Tlamimilopa Phase of Teotihuacan is the period of greatest splendor and laid the groundwork for the Xolalpan Phase which shows Teotihuacan having the greatest influence in Mesoamerica. Considering that the entrada took place only four years after Spearthrower Owl's ascension to the throne, he could have decided on a different course of action for Teotihuacan - a more aggressive one to increase its prestige, power and influence. One thing to take into consideration though is that while there is record of Siyah K'ak' having been at El Peru eight days before arriving at Tikal, it seems that Tikal was their ultimate destination, the defeat of Uaxactun probably having more to do with its connection (and closeness) to Tikal. Was it a case of the current king of Tikal, Chak Tok Ich'aak I, handing off power to a non-related, foreign power? More than likely not. Could be that Chak Tok Ich'aak I angered Spearthrower Owl and it was a mission of retribution? Personally, I would find that scenario a lot more likely.

As for evidence...that's the hard part. I find it highly suggestive that it was a military takeover due to the fact that Chak Tok Ich'aak met his death on the same day Siyah K'ak' entered Tikal. Normally, when one Maya king would attack another, if they managed to capture the king himself he would be sacrificed. I'm not entirely sure if Teotihuacan followed that same type of behaviour, but that would be my guess as to what took place. That in addition to the fact that most of Tikal's monuments were destroyed definitely seems to lean towards a hostile takeover.

The veneration of Chak Tok Ich'aak I really isn't that much of a surprise. This is actually what I consider to be the hardest part to explain so I'll try to be as concise as possible. LOL The veneration had more to do with continuity and tradition than anything else. Egyptian kings fell into the same patterns too. It was to connect the current kings to the previous dynasty and maintain their right to be kings of Tikal. If Spearthrower Owl did in fact marry a lady of Tikal one of the reasonings behind that could have been to form a marital connection with the city and provide a foundation for his son to become king. I think the veneration aspect really depended upon who was king and what political agenda they had during their reign. A good example of this would be Jasaw Chan K'awiil I including the anniversary of Spearthrower Owl's death in his celebrations in 695. Jasaw Chan K'awiil I was not descended from Spearthrower Owl's dynasty, but it suited his political agenda and the general view of Tikal he was trying to promote, connecting his reign (a revitalization of Tikal's power) to a prior powerful dynastic reign. It rather seems to come down to what propaganda was best suited at that time. LOL

*eyes crossing* I hope that all made sense. LOL Let me know if you need any clarification or if this made more questions arise. *g*