Could a vampire bond another simply by spiking a blood pack? Could a friendly vampire then just be bonded to just all the players after handing them all some without them even realising?
It is a possibility.
According to the VtM20 rulebook, it takes three "drinks" (though these can be as small as sips) over three separate nights to create a fully formed Blood Bond. This could very much work to establish the first level of a Blood Bond, and the tendencies for Kindred to "coincidentally frequent places where he might show up," could lead to a similar repeat occurrence in order to strengthen the bond.
However, given the repeated thoughts of, dreams of, and "chance encounters" with, not to mention feelings for the Kindred who spiked the blood bag, it seems to me that most Kindred would become aware of the situation rather quickly, unless they were completely without knowledge of Blood Bonds. Depending on how strong the spiker is (or how much backup/clout said vampire has), retribution seems more likely than getting a full 3 drinks into the PC's. The Kindred performing the spiking would want to develop a plan to get the PC's to take the second drink as soon as possible, in order to benefit from the increased influence and rapport gained - which would reduce the chances of retribution, and increase the likelihood of getting that third sip.
The fluff text in several rule books, in several editions of the game, warns kindred of protecting their blood supply from such tampering, and the system goes so far as to provide a background stat for a herd that you maintain that has not been so tampered with.
This is the basis of the taboo against feeding on Ghouls.
I cannot remember a specific reference to a blood pack being used as the delivery method for tampered blood. At the same time, I do not recall blood packs being mentioned as a common source of vitae for kindred either.
Also keep in mind that the effects of drinking another kindred's blood are not entirely obvious unless you are monitoring yourself diligently for them. It would not seem unnatural to you that your feelings for the other kindred had changed. That's kind of the point of the effect. If it were so easily detected, and therefor countered against, it wouldn't be nearly the threat it is.