Pathfinder runs on a d20 system, which is a high variance system (you can fail by a lot or succeed by a lot). This is a result of the fact that a d20 dice roll uniformly selects a value in a range of 20 values, with an expected roll of 10.5.
Systems like Shadowrun and GURPS tend to run based on 3d6 or sums of bernoulli random variables based on 1d6 rolls (5 or 6 'hits', 1/3 chance of 'hit'). These types of rolls have tighter distributions with lower variance (you most likely won't be far off from the average on a regular basis).
A problem with this is that CR as a measure of difficulty fails to account for action economy. It instead takes into account the modifiers that shift these 1-20 ranges up and down with respect to AC, Attack, Will Saves, and certain special talents. DCs, AC, and etc simply bisect this range of values into Fail and Success regions. As you slide this range by a +1 or -1 modifier, you change the probability of success by ±5%.
In the other systems, a change of +1 or -1, this change is is curved against where you already were. See the image below for probability of success for different target rolls:
As you can see, the impact of a 1-point shift isn't as significant in Pathfinder as it is in the other systems. However, CR is measured in these 1-point shifts (sometimes further skewed by special abilities), but in the early levels at least, it ends up boiling down to modifiers and HP.
Consider a CR 4 creature: The Giant Stag Beetle
This creature is designed to be fought by a party of four Level 4 adventurers for a "fair" fight. By level 4, a fighter will have +4 Base Attack and +3 STR Bonus (most likely more). On average, a fighter will hit this Beetle's 17 AC.
Consider now that there are four other players considered to be on "level ground with that fighter". Every full rotation of turns, this beetle will be taking two attacks on average, up to four total, and only dishing out one attack of it's own. To accommodate for this quarter-as-many-attacks, it's been given a 2d8 bite which is the equivalent of two longsword slashes. This still leaves it as doing half-as-much on average as a party of four and with a much higher variance.
If you continue to investigate it, you'll see that this beetle roughly equates to two level 4 fighters fighting within a 10-by-10 square and both targeting the same target each turn, in health and damage, but not in feats and special abilities. This encounter is already at a disadvantage in a combat that was meant to be fair. Four against two.
Now, consider that you've given each of your players the opportunity to gain additional actions per turn. This means additional attacks or additional actions. You've turned an already lopsided battle from a four-versus-two equivalent to an up-to eight-versus-two equivalent. You could very well defeat this Giant Stag Beetle before it even gets a turn with additional actions on each player.
These are the kinds of imbalances that additional actions cause. Even without homebrew, it already happens!
If there is a crafter in your party, he can craft Boots of Speed at around Level 3-4. The cost to make a pair of boots is 3k, which is half of the expected wealth of a level 4 player. The expectation that a player will have no item that cost them more than half their total wealth is being respected, but they are now all gaining an additional attack each turn.
Even one player having a pair of these boots lessens the impact of a CR 4 encounter as described above and now we can see that it's perfectly possible for it to happen and fairly commonly.