Because there can be only one predicate per clause and that is the verb profitēbantur, while aggressī is a participle, describing senēs and standing in for the clause "postquam aggressī sunt": "..when, having approached him, some old men started saying that they were...". To see the effect of two predicates per clause, consider En. "suddenly a man approached asked me". The cum here [is inverse][1]:

2.E.4. In inverted clauses, the principal sentence determining the time of the clause, cum ( = quo tempore) having the force of a relative; cum with the indic. always following the principal sentence; never in oblique discourse [..] principal sentence often with jam, vix, vixdum, nondum, tantum quod, and commodum; cum often with subito, repente, sometimes interim, tamen, etiamtum.

It can be reversed as "cum paene ascendisset, senēs eum aggressī profitēbantur". I must say that considering the part about lēgātōrum mōre ("in the guise of envoys") is already expressed, I struggle to imagine what is supposed to follow profitēbantur. [1]: http://alatius.com/ls/index.php?l=cum

Because there can be only one predicate per clause and that is the verb profitēbantur, while aggressī is a participle, describing senēs and standing in for the clause "postquam aggressī sunt": "..when, having approached him, some old men started saying that they were...". To see the effect of two predicates per clause, consider En. "suddenly a man approached asked me". The cum here is inverse:

2.E.4. In inverted clauses, the principal sentence determining the time of the clause, cum ( = quo tempore) having the force of a relative; cum with the indic. always following the principal sentence; never in oblique discourse [..] principal sentence often with jam, vix, vixdum, nondum, tantum quod, and commodum; cum often with subito, repente, sometimes interim, tamen, etiamtum.

It can be reversed as "cum paene ascendisset, senēs eum aggressī profitēbantur". I must say that considering the part about lēgātōrum mōre ("in the guise of envoys") is already expressed, I struggle to imagine what is supposed to follow profitēbantur.

Because there can be only one predicate per clause and that is the verb profitēbantur, while aggressī is a participle, describing senēs and standing in for the clause "postquam aggressī sunt": "..when, having approached him, some old men started saying that they were...". To see the effect of two predicates per clause, consider En. "suddenly a man approached asked me". The cum here [is inverse][1]:

4. In inverted clauses, the principal sentence determining the time of the clause, cum ( = quo tempore) having the force of a relative; cum with the indic. always following the principal sentence; never in oblique discourse [..] principal sentence often with jam, vix, vixdum, nondum, tantum quod, and commodum; cum often with subito, repente, sometimes interim, tamen, etiamtum.

It can be reversed as "cum paene ascendisset, senēs eum aggressī profitēbantur". I must say that considering the part about lēgātōrum mōre ("in the guise of envoys") is already expressed, I struggle to imagine what is supposed to follow profitēbantur. [1]: http://alatius.com/ls/index.php?l=cum

Because there can be only one predicate per clause and that is the verb profitēbantur, while aggressī is a participle, describing senēs and standing in for the clause "postquam aggressī sunt": "..when, having approached him, some old men started saying that they were...". To see the effect of two predicates per clause, consider En. "suddenly a man approached asked me". The cum here [is inverse][1]:

2.E.4. In inverted clauses, the principal sentence determining the time of the clause, cum ( = quo tempore) having the force of a relative; cum with the indic. always following the principal sentence; never in oblique discourse [..] principal sentence often with jam, vix, vixdum, nondum, tantum quod, and commodum; cum often with subito, repente, sometimes interim, tamen, etiamtum.

It can be reversed as "cum paene ascendisset, senēs eum aggressī profitēbantur". I must say that considering the part about lēgātōrum mōre ("in the guise of envoys") is already expressed, I struggle to imagine what is supposed to follow profitēbantur. [1]: http://alatius.com/ls/index.php?l=cum